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https://github.com/Idnan/bash-guide.git
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1181 lines
27 KiB
Markdown
1181 lines
27 KiB
Markdown
<p align="center">
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<img src="https://cloud.githubusercontent.com/assets/2059754/24601246/753a7f36-1858-11e7-9d6b-7a0e64fb27f7.png" alt="bash logo"/>
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</p>
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## Table of Contents
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1. [Basic Operations](#1-basic-operations)
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1.1. [File Operations](#11-file-operations)
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1.2. [Text Operations](#12-text-operations)
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1.3. [Directory Operations](#13-directory-operations)
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1.4. [SSH, System Info & Network Operations](#14-ssh-system-info--network-operations)
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1.5. [Process Monitoring Operations](#15-process-monitoring-operations)
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2. [Basic Shell Programming](#2-basic-shell-programming)
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2.1. [Variables](#21-variables)
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2.2 [Array](#22-array)
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2.3. [String Substitution](#23-string-substitution)
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2.4. [Functions](#24-functions)
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2.5. [Conditionals](#25-conditionals)
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2.6. [Loops](#26-loops)
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3. [Tricks](#3-tricks)
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4. [Debugging](#4-debugging)
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# 1. Basic Operations
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### a. `export`
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Displays all environment variables. If you want to get details of a specific variable, use `echo $VARIABLE_NAME`.
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```bash
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export
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```
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Example:
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```bash
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$ export
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AWS_HOME=/Users/adnanadnan/.aws
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LANG=en_US.UTF-8
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LC_CTYPE=en_US.UTF-8
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LESS=-R
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$ echo $AWS_HOME
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/Users/adnanadnan/.aws
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```
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### b. `whatis`
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whatis shows description for user commands, system calls, library functions, and others in manual pages
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```bash
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whatis something
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```
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Example:
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```bash
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$ whatis bash
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bash (1) - GNU Bourne-Again SHell
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```
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### c. `whereis`
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whereis searches for executables, source files, and manual pages using a database built by system automatically.
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```bash
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whereis name
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```
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Example:
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```bash
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$ whereis php
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/usr/bin/php
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```
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### d. `which`
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which searches for executables in the directories specified by the environment variable PATH. This command will print the full path of the executable(s).
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```bash
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which program_name
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```
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Example:
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```bash
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$ which php
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/c/xampp/php/php
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```
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### e. clear
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Clears content on window.
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## 1.1. File Operations
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<table>
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<tr>
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<td><a href="#a-cat">cat</a></td>
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<td><a href="#b-chmod">chmod</a></td>
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<td><a href="#c-chown">chown</a></td>
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<td><a href="#d-cp">cp</a></td>
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<td><a href="#e-diff">diff</a></td>
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<td><a href="#f-file">file</a></td>
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<td><a href="#g-find">find</a></td>
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<td><a href="#h-gunzip">gunzip</a></td>
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<td><a href="#i-gzcat">gzcat</a></td>
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<td><a href="#j-gzip">gzip</a></td>
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<td><a href="#k-head">head</a></td>
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</tr>
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<tr>
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<td><a href="#l-lpq">lpq</a></td>
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<td><a href="#m-lpr">lpr</a></td>
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<td><a href="#n-lprm">lprm</a></td>
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<td><a href="#o-ls">ls</a></td>
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<td><a href="#p-more">more</a></td>
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<td><a href="#q-mv">mv</a></td>
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<td><a href="#r-rm">rm</a></td>
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<td><a href="#s-tail">tail</a></td>
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<td><a href="#t-touch">touch</a></td>
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</tr>
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</table>
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### a. `cat`
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It can be used for the following purposes under UNIX or Linux.
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* Display text files on screen
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* Copy text files
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* Combine text files
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* Create new text files
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```bash
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cat filename
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cat file1 file2
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cat file1 file2 > newcombinedfile
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cat < file1 > file2 #copy file1 to file2
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```
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### b. `chmod`
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The chmod command stands for "change mode" and allows you to change the read, write, and execute permissions on your files and folders. For more information on this command check this [link](https://ss64.com/bash/chmod.html).
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```bash
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chmod -options filename
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```
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### c. `chown`
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The chown command stands for "change owner", and allows you to change the owner of a given file or folder, which can be a user and a group. Basic usage is simple forward first comes the user (owner), and then the group, delimited by a colon.
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```bash
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chown -options user:group filename
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```
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### d. `cp`
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Copies a file from one location to other.
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```bash
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cp filename1 filename2
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```
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Where `filename1` is the source path to the file and `filename2` is the destination path to the file.
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### e. `diff`
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Compares files, and lists their differences.
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```bash
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diff filename1 filename2
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```
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### f. `file`
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Determine file type.
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```bash
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file filename
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```
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Example:
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```bash
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$ file index.html
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index.html: HTML document, ASCII text
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```
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### g. `find`
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Find files in directory
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```bash
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find directory options pattern
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```
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Example:
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```bash
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$ find . -name README.md
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$ find /home/user1 -name '*.png'
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```
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### h. `gunzip`
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Un-compresses files compressed by gzip.
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```bash
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gunzip filename
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```
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### i. `gzcat`
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Lets you look at gzipped file without actually having to gunzip it.
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```bash
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gzcat filename
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```
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### j. `gzip`
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Compresses files.
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```bash
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gzip filename
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```
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### k. `head`
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Outputs the first 10 lines of file
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```bash
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head filename
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```
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### l. `lpq`
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Check out the printer queue.
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```bash
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lpq
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```
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Example:
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```bash
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$ lpq
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Rank Owner Job File(s) Total Size
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active adnanad 59 demo 399360 bytes
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1st adnanad 60 (stdin) 0 bytes
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```
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### m. `lpr`
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Print the file.
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```bash
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lpr filename
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```
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### n. `lprm`
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Remove something from the printer queue.
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```bash
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lprm jobnumber
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```
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### o. `ls`
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Lists your files. `ls` has many options: `-l` lists files in 'long format', which contains the exact size of the file, who owns the file, who has the right to look at it, and when it was last modified. `-a` lists all files, including hidden files. For more information on this command check this [link](https://ss64.com/bash/ls.html).
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```bash
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ls option
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```
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Example:
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<pre>
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$ ls -la
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rwxr-xr-x 33 adnan staff 1122 Mar 27 18:44 .
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drwxrwxrwx 60 adnan staff 2040 Mar 21 15:06 ..
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-rw-r--r--@ 1 adnan staff 14340 Mar 23 15:05 .DS_Store
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-rw-r--r-- 1 adnan staff 157 Mar 25 18:08 .bumpversion.cfg
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-rw-r--r-- 1 adnan staff 6515 Mar 25 18:08 .config.ini
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-rw-r--r-- 1 adnan staff 5805 Mar 27 18:44 .config.override.ini
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drwxr-xr-x 17 adnan staff 578 Mar 27 23:36 .git
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-rwxr-xr-x 1 adnan staff 2702 Mar 25 18:08 .gitignore
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</pre>
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### p. `more`
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Shows the first part of a file (move with space and type q to quit).
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```bash
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more filename
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```
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### q. `mv`
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Moves a file from one location to other.
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```bash
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mv filename1 filename2
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```
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Where `filename1` is the source path to the file and `filename2` is the destination path to the file.
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Also it can be used for rename a file.
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```bash
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mv old_name new_name
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```
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### r. `rm`
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Removes a file. Using this command on a directory gives you an error.
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`rm: directory: is a directory`
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To remove a directory you have to pass `-r` which will remove the content of the directory recursively. Optionally you can use `-f` flag to force the deletion i.e. without any confirmations etc.
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```bash
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rm filename
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```
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### s. `tail`
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Outputs the last 10 lines of file. Use `-f` to output appended data as the file grows.
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```bash
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tail filename
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```
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### t. `touch`
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Updates access and modification time stamps of your file. If it doesn't exists, it'll be created.
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```bash
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touch filename
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```
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Example:
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```bash
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$ touch trick.md
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```
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## 1.2. Text Operations
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<table>
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<tr>
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<td><a href="#a-awk">awk</a></td>
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<td><a href="#b-cut">cut</a></td>
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<td><a href="#c-echo">echo</a></td>
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<td><a href="#d-egrep">egrep</a></td>
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<td><a href="#e-fgrep">fgrep</a></td>
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<td><a href="#f-fmt">fmt</a></td>
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<td><a href="#g-grep">grep</a></td>
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<td><a href="#h-nl">nl</a></td>
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<td><a href="#i-sed">sed</a></td>
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<td><a href="#j-sort">sort</a></td>
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</tr>
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<tr>
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<td><a href="#k-tr">tr</a></td>
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<td><a href="#l-uniq">uniq</a></td>
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<td><a href="#m-wc">wc</a></td>
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</tr>
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</table>
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### a. `awk`
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awk is the most useful command for handling text files. It operates on an entire file line by line. By default it uses whitespace to separate the fields. The most common syntax for awk command is
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```bash
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awk '/search_pattern/ { action_to_take_if_pattern_matches; }' file_to_parse
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```
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Lets take following file `/etc/passwd`. Here's the sample data that this file contains:
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```
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root:x:0:0:root:/root:/usr/bin/zsh
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daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
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bin:x:2:2:bin:/bin:/usr/sbin/nologin
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sys:x:3:3:sys:/dev:/usr/sbin/nologin
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sync:x:4:65534:sync:/bin:/bin/sync
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```
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So now lets get only username from this file. Where `-F` specifies that on which base we are going to separate the fields. In our case it's `:`. `{ print $1 }` means print out the first matching field.
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```bash
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awk -F':' '{ print $1 }' /etc/passwd
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```
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After running the above command you will get following output.
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```
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root
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daemon
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bin
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sys
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sync
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```
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For more detail on how to use `awk`, check following [link](https://www.cyberciti.biz/faq/bash-scripting-using-awk).
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### b. `cut`
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Remove sections from each line of files
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*example.txt*
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```bash
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red riding hood went to the park to play
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```
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*show me columns 2 , 7 , and 9 with a space as a separator*
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```bash
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cut -d " " -f2,7,9 example.txt
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```
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```bash
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riding park play
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```
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### c. `echo`
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Display a line of text
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*display "Hello World"*
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```bash
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echo Hello World
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```
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```bash
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Hello World
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```
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*display "Hello World" with newlines between words*
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```bash
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echo -ne "Hello\nWorld\n"
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```
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```bash
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Hello
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World
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```
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### d. `egrep`
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Print lines matching a pattern - Extended Expression (alias for: 'grep -E')
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*example.txt*
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```bash
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Lorem ipsum
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dolor sit amet,
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consetetur
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sadipscing elitr,
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sed diam nonumy
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eirmod tempor
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invidunt ut labore
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et dolore magna
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aliquyam erat, sed
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diam voluptua. At
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vero eos et
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accusam et justo
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duo dolores et ea
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rebum. Stet clita
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kasd gubergren,
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no sea takimata
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sanctus est Lorem
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ipsum dolor sit
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amet.
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```
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*display lines that have either "Lorem" or "dolor" in them.*
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```bash
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egrep '(Lorem|dolor)' example.txt
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or
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grep -E '(Lorem|dolor)' example.txt
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```
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```bash
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Lorem ipsum
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dolor sit amet,
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et dolore magna
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duo dolores et ea
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sanctus est Lorem
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ipsum dolor sit
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```
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### e. `fgrep`
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Print lines matching a pattern - FIXED pattern matching (alias for: 'grep -F')
|
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*example.txt*
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```bash
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Lorem ipsum
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dolor sit amet,
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consetetur
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sadipscing elitr,
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sed diam nonumy
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eirmod tempor
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foo (Lorem|dolor)
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invidunt ut labore
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et dolore magna
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aliquyam erat, sed
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diam voluptua. At
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vero eos et
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accusam et justo
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duo dolores et ea
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rebum. Stet clita
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kasd gubergren,
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no sea takimata
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sanctus est Lorem
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ipsum dolor sit
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amet.
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```
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*Find the exact string '(Lorem|dolor)' in example.txt*
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```bash
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fgrep '(Lorem|dolor)' example.txt
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or
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grep -F '(Lorem|dolor)' example.txt
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```
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```bash
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foo (Lorem|dolor)
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```
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### f. `fmt`
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Simple optimal text formatter
|
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|
|
*example: example.txt (1 line)*
|
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```bash
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Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.
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```
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*output the lines of example.txt to 20 character width*
|
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```bash
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cat example.txt | fmt -w 20
|
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```
|
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```bash
|
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Lorem ipsum
|
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dolor sit amet,
|
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consetetur
|
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sadipscing elitr,
|
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sed diam nonumy
|
|
eirmod tempor
|
|
invidunt ut labore
|
|
et dolore magna
|
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aliquyam erat, sed
|
|
diam voluptua. At
|
|
vero eos et
|
|
accusam et justo
|
|
duo dolores et ea
|
|
rebum. Stet clita
|
|
kasd gubergren,
|
|
no sea takimata
|
|
sanctus est Lorem
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|
ipsum dolor sit
|
|
amet.
|
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```
|
|
|
|
### g. `grep`
|
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Looks for text inside files. You can use grep to search for lines of text that match one or many regular expressions, and outputs only the matching lines.
|
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```bash
|
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grep pattern filename
|
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```
|
|
Example:
|
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```bash
|
|
$ grep admin /etc/passwd
|
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_kadmin_admin:*:218:-2:Kerberos Admin Service:/var/empty:/usr/bin/false
|
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_kadmin_changepw:*:219:-2:Kerberos Change Password Service:/var/empty:/usr/bin/false
|
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_krb_kadmin:*:231:-2:Open Directory Kerberos Admin Service:/var/empty:/usr/bin/false
|
|
```
|
|
You can also force grep to ignore word case by using `-i` option. `-r` can be used to search all files under the specified directory, for example:
|
|
```bash
|
|
$ grep -r admin /etc/
|
|
```
|
|
And `-w` to search for words only. For more detail on `grep`, check following [link](https://www.cyberciti.biz/faq/grep-in-bash).
|
|
|
|
### h. `nl`
|
|
Number lines of files
|
|
|
|
*example.txt*
|
|
```bash
|
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Lorem ipsum
|
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dolor sit amet,
|
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consetetur
|
|
sadipscing elitr,
|
|
sed diam nonumy
|
|
eirmod tempor
|
|
invidunt ut labore
|
|
et dolore magna
|
|
aliquyam erat, sed
|
|
diam voluptua. At
|
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vero eos et
|
|
accusam et justo
|
|
duo dolores et ea
|
|
rebum. Stet clita
|
|
kasd gubergren,
|
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no sea takimata
|
|
sanctus est Lorem
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ipsum dolor sit
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amet.
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```
|
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|
|
*show example.txt with line numbers*
|
|
```bash
|
|
nl -s". " example.txt
|
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```
|
|
```bash
|
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1. Lorem ipsum
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2. dolor sit amet,
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3. consetetur
|
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4. sadipscing elitr,
|
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5. sed diam nonumy
|
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6. eirmod tempor
|
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7. invidunt ut labore
|
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8. et dolore magna
|
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9. aliquyam erat, sed
|
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10. diam voluptua. At
|
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11. vero eos et
|
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12. accusam et justo
|
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13. duo dolores et ea
|
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14. rebum. Stet clita
|
|
15. kasd gubergren,
|
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16. no sea takimata
|
|
17. sanctus est Lorem
|
|
18. ipsum dolor sit
|
|
19. amet.
|
|
```
|
|
|
|
### i. `sed`
|
|
Stream editor for filtering and transforming text
|
|
|
|
*example.txt*
|
|
```bash
|
|
Hello This is a Test 1 2 3 4
|
|
```
|
|
|
|
*replace all spaces with hyphens*
|
|
```bash
|
|
sed 's/ /-/g' example.txt
|
|
```
|
|
```bash
|
|
Hello-This-is-a-Test-1-2-3-4
|
|
```
|
|
|
|
*replace all digits with "d"*
|
|
```bash
|
|
sed 's/[0-9]/d/g' example.txt
|
|
```
|
|
```bash
|
|
Hello This is a Test d d d d
|
|
```
|
|
|
|
### j. `sort`
|
|
Sort lines of text files
|
|
|
|
*example.txt*
|
|
```bash
|
|
f
|
|
b
|
|
c
|
|
g
|
|
a
|
|
e
|
|
d
|
|
```
|
|
|
|
*sort example.txt*
|
|
```bash
|
|
sort example.txt
|
|
```
|
|
```bash
|
|
a
|
|
b
|
|
c
|
|
d
|
|
e
|
|
f
|
|
g
|
|
```
|
|
|
|
*randomize a sorted example.txt*
|
|
```bash
|
|
sort example.txt | sort -R
|
|
```
|
|
```bash
|
|
b
|
|
f
|
|
a
|
|
c
|
|
d
|
|
g
|
|
e
|
|
```
|
|
|
|
### k. `tr`
|
|
Translate or delete characters
|
|
|
|
*example.txt*
|
|
```bash
|
|
Hello World Foo Bar Baz!
|
|
```
|
|
|
|
*take all lower case letters and make them upper case*
|
|
```bash
|
|
cat example.txt | tr 'a-z' 'A-Z'
|
|
```
|
|
```bash
|
|
HELLO WORLD FOO BAR BAZ!
|
|
```
|
|
|
|
*take all spaces and make them into newlines*
|
|
```bash
|
|
cat example.txt | tr ' ' '\n'
|
|
```
|
|
```bash
|
|
Hello
|
|
World
|
|
Foo
|
|
Bar
|
|
Baz!
|
|
```
|
|
|
|
### l. `uniq`
|
|
Report or omit repeated lines
|
|
|
|
*example.txt*
|
|
```bash
|
|
a
|
|
a
|
|
b
|
|
a
|
|
b
|
|
c
|
|
d
|
|
c
|
|
```
|
|
|
|
*show only unique lines of example.txt (first you need to sort it, otherwise it won't see the overlap)*
|
|
```bash
|
|
sort example.txt | uniq
|
|
```
|
|
```bash
|
|
a
|
|
b
|
|
c
|
|
d
|
|
```
|
|
|
|
*show the unique items for each line, and tell me how many instances it found*
|
|
```bash
|
|
sort example.txt | uniq -c
|
|
```
|
|
```bash
|
|
3 a
|
|
2 b
|
|
2 c
|
|
1 d
|
|
```
|
|
|
|
### m. `wc`
|
|
Tells you how many lines, words and characters there are in a file.
|
|
```bash
|
|
wc filename
|
|
```
|
|
Example:
|
|
```bash
|
|
$ wc demo.txt
|
|
7459 15915 398400 demo.txt
|
|
```
|
|
Where `7459` is lines, `15915` is words and `398400` is characters.
|
|
|
|
## 1.3. Directory Operations
|
|
|
|
<table>
|
|
<tr>
|
|
<td><a href="#a-cd">cd</a></td>
|
|
<td><a href="#b-mkdir">mkdir</a></td>
|
|
<td><a href="#c-pwd">pwd</a></td>
|
|
</tr>
|
|
</table>
|
|
|
|
### a. `cd`
|
|
Moves you from one directory to other. Running this
|
|
```bash
|
|
$ cd
|
|
```
|
|
moves you to home directory. This command accepts an optional `dirname`, which moves you to that directory.
|
|
```bash
|
|
cd dirname
|
|
```
|
|
|
|
### b. `mkdir`
|
|
Makes a new directory.
|
|
```bash
|
|
mkdir dirname
|
|
```
|
|
|
|
### c. `pwd`
|
|
Tells you which directory you currently are in.
|
|
```bash
|
|
pwd
|
|
```
|
|
|
|
## 1.4. SSH, System Info & Network Operations
|
|
|
|
<table>
|
|
<tr>
|
|
<td><a href="#a-bg">bg</a></td>
|
|
<td><a href="#b-cal">cal</a></td>
|
|
<td><a href="#c-date">date</a></td>
|
|
<td><a href="#d-df">df</a></td>
|
|
<td><a href="#e-dig">dig</a></td>
|
|
<td><a href="#f-du">du</a></td>
|
|
<td><a href="#g-fg">fg</a></td>
|
|
<td><a href="#h-finger">finger</a></td>
|
|
<td><a href="#i-jobs">jobs</a></td>
|
|
<td><a href="#j-last">last</a></td>
|
|
</tr>
|
|
<tr>
|
|
<td><a href="#k-man">man</a></td>
|
|
<td><a href="#l-passwd">passwd</a></td>
|
|
<td><a href="#m-ping">ping</a></td>
|
|
<td><a href="#n-ps">ps</a></td>
|
|
<td><a href="#o-quota">quota</a></td>
|
|
<td><a href="#p-scp">scp</a></td>
|
|
<td><a href="#q-ssh">ssh</a></td>
|
|
<td><a href="#r-top">top</a></td>
|
|
<td><a href="#s-uname">uname</a></td>
|
|
<td><a href="#t-uptime">uptime</a></td>
|
|
</tr>
|
|
<tr>
|
|
<td><a href="#u-w">w</a></td>
|
|
<td><a href="#v-wget">wget</a></td>
|
|
<td><a href="#w-whoami">whoami</a></td>
|
|
<td><a href="#x-whois">whois</a></td>
|
|
</tr>
|
|
</table>
|
|
|
|
### a. `bg`
|
|
Lists stopped or background jobs; resume a stopped job in the background.
|
|
|
|
### b. `cal`
|
|
Shows the month's calendar.
|
|
|
|
### c. `date`
|
|
Shows the current date and time.
|
|
|
|
### d. `df`
|
|
Shows disk usage.
|
|
|
|
### e. `dig`
|
|
Gets DNS information for domain.
|
|
```bash
|
|
dig domain
|
|
```
|
|
|
|
### f. `du`
|
|
Shows the disk usage of files or directories. For more information on this command check this [link](http://www.linfo.org/du.html)
|
|
```bash
|
|
du [option] [filename|directory]
|
|
```
|
|
Options:
|
|
- `-h` (human readable) Displays output it in kilobytes (K), megabytes (M) and gigabytes (G).
|
|
- `-s` (supress or summarize) Outputs total disk space of a directory and supresses reports for subdirectories.
|
|
|
|
Example:
|
|
```bash
|
|
du -sh pictures
|
|
1.4M pictures
|
|
```
|
|
|
|
### g. `fg`
|
|
Brings the most recent job in the foreground.
|
|
|
|
### h. `finger`
|
|
Displays information about user.
|
|
```bash
|
|
finger username
|
|
```
|
|
### i. `jobs`
|
|
Lists the jobs running in the background, giving the job number.
|
|
|
|
### j. `last`
|
|
Lists your last logins of specified user.
|
|
```bash
|
|
last yourUsername
|
|
```
|
|
|
|
### k. `man`
|
|
Shows the manual for specified command.
|
|
```bash
|
|
man command
|
|
```
|
|
|
|
### l. `passwd`
|
|
Allows the current logged user to change his password.
|
|
|
|
### m. `ping`
|
|
Pings host and outputs results.
|
|
```bash
|
|
ping host
|
|
```
|
|
|
|
### n. `ps`
|
|
Lists your processes.
|
|
```bash
|
|
ps -u yourusername
|
|
```
|
|
|
|
### o. `quota`
|
|
Shows what your disk quota is.
|
|
```bash
|
|
quota -v
|
|
```
|
|
|
|
### p. `scp`
|
|
Transfer files between a local host and a remote host or between two remote hosts.
|
|
|
|
*copy from local host to remote host*
|
|
```bash
|
|
scp source_file user@host:directory/target_file
|
|
```
|
|
*copy from remote host to local host*
|
|
```bash
|
|
scp user@host:directory/source_file target_file
|
|
scp -r user@host:directory/source_folder target_folder
|
|
```
|
|
This command also accepts an option `-P` that can be used to connect to specific port.
|
|
```bash
|
|
scp -P port user@host:directory/source_file target_file
|
|
```
|
|
|
|
### q. `ssh`
|
|
ssh (SSH client) is a program for logging into and executing commands on a remote machine.
|
|
```bash
|
|
ssh user@host
|
|
```
|
|
This command also accepts an option `-p` that can be used to connect to specific port.
|
|
```bash
|
|
ssh -p port user@host
|
|
```
|
|
|
|
### r. `top`
|
|
Displays your currently active processes.
|
|
|
|
### s. `uname`
|
|
Shows kernel information.
|
|
```bash
|
|
uname -a
|
|
```
|
|
|
|
### t. `uptime`
|
|
Shows current uptime.
|
|
|
|
### u. `w`
|
|
Displays who is online.
|
|
|
|
### v. `wget`
|
|
Downloads file.
|
|
```bash
|
|
wget file
|
|
```
|
|
|
|
### w. `whoami`
|
|
Return current logged in username.
|
|
|
|
### x. `whois`
|
|
Gets whois information for domain.
|
|
```bash
|
|
whois domain
|
|
```
|
|
|
|
## 1.5. Process Monitoring Operations
|
|
|
|
<table>
|
|
<tr>
|
|
<td><a href="#a-kill">kill</a></td>
|
|
<td><a href="#b-killall">killall</a></td>
|
|
<td><a href="#c-&">&</a></td>
|
|
<td><a href="#d-nohup">nohup</a></td>
|
|
</tr>
|
|
</table>
|
|
|
|
### a. `kill`
|
|
Kills (ends) the processes with the ID you gave.
|
|
```bash
|
|
kill PID
|
|
```
|
|
|
|
### b. `killall`
|
|
Kill all processes with the name.
|
|
```bash
|
|
killall processname
|
|
```
|
|
|
|
### c. &
|
|
The `&` symbol instructs the command to run as a background process in a subshell.
|
|
```bash
|
|
command &
|
|
```
|
|
|
|
### d. `nohup`
|
|
nohup stands for "No Hang Up". This allows to run command/process or shell script that can continue running in the background after you log out from a shell.
|
|
```bash
|
|
nohup command
|
|
```
|
|
Combine it with `&` to create background processes
|
|
```bash
|
|
nohup command &
|
|
```
|
|
|
|
# 2. Basic Shell Programming
|
|
|
|
|
|
The first line that you will write in bash script files is called `shebang`. This line in any script determines the script's ability to be executed like a standalone executable without typing sh, bash, python, php etc beforehand in the terminal.
|
|
|
|
```bash
|
|
#!/bin/bash
|
|
```
|
|
|
|
## 2.1. Variables
|
|
|
|
Creating variables in bash is similar to other languages. There are no data types. A variable in bash can contain a number, a character, a string of characters, etc. You have no need to declare a variable, just assigning a value to its reference will create it.
|
|
|
|
Example:
|
|
```bash
|
|
str="hello world"
|
|
```
|
|
|
|
The above line creates a variable `str` and assigns "hello world" to it. The value of variable is retrieved by putting the `$` in the beginning of variable name.
|
|
|
|
Example:
|
|
```bash
|
|
echo $str # hello world
|
|
```
|
|
## 2.2. Array
|
|
Like other languages bash has also arrays. An array is variable containing multiple values. There's no maximum limit on the size of array. Array in bash are zero based. The first element is indexed with element 0. There are several ways for creating arrays in bash. Which are given below.
|
|
|
|
Examples:
|
|
```bash
|
|
array[0] = val
|
|
array[1] = val
|
|
array[2] = val
|
|
array=([2]=val [0]=val [1]=val)
|
|
array=(val val val)
|
|
```
|
|
To display a value at specific index use following syntax:
|
|
|
|
```bash
|
|
${array[i]} # where i is the index
|
|
```
|
|
|
|
If no index is supplied, array element 0 is assumed. To find out how many values there are in the array use the following syntax:
|
|
|
|
```bash
|
|
${#array[@]}
|
|
```
|
|
|
|
Bash has also support for the ternary conditions. Check some examples below.
|
|
|
|
```bash
|
|
${varname:-word} # if varname exists and isn't null, return its value; otherwise return word
|
|
${varname:=word} # if varname exists and isn't null, return its value; otherwise set it word and then return its value
|
|
${varname:+word} # if varname exists and isn't null, return word; otherwise return null
|
|
${varname:offset:length} # performs substring expansion. It returns the substring of $varname starting at offset and up to length characters
|
|
```
|
|
|
|
## 2.3 String Substitution
|
|
|
|
Check some of the syntax on how to manipulate strings
|
|
|
|
```bash
|
|
${variable#pattern} # if the pattern matches the beginning of the variable's value, delete the shortest part that matches and return the rest
|
|
${variable##pattern} # if the pattern matches the beginning of the variable's value, delete the longest part that matches and return the rest
|
|
${variable%pattern} # if the pattern matches the end of the variable's value, delete the shortest part that matches and return the rest
|
|
${variable%%pattern} # if the pattern matches the end of the variable's value, delete the longest part that matches and return the rest
|
|
${variable/pattern/string} # the longest match to pattern in variable is replaced by string. Only the first match is replaced
|
|
${variable//pattern/string} # the longest match to pattern in variable is replaced by string. All matches are replaced
|
|
${#varname} # returns the length of the value of the variable as a character string
|
|
```
|
|
|
|
## 2.4. Functions
|
|
As in almost any programming language, you can use functions to group pieces of code in a more logical way or practice the divine art of recursion. Declaring a function is just a matter of writing function my_func { my_code }. Calling a function is just like calling another program, you just write its name.
|
|
|
|
```bash
|
|
functname() {
|
|
shell commands
|
|
}
|
|
```
|
|
|
|
Example:
|
|
```bash
|
|
#!/bin/bash
|
|
function hello {
|
|
echo world!
|
|
}
|
|
hello
|
|
|
|
function say {
|
|
echo $1
|
|
}
|
|
say "hello world!"
|
|
```
|
|
|
|
When you run the above example the `hello` function will output "world!". The above two functions `hello` and `say` are identical. The main difference is function `say`. This function, prints the first argument it receives. Arguments, within functions, are treated in the same manner as arguments given to the script.
|
|
|
|
## 2.5. Conditionals
|
|
|
|
The conditional statement in bash is similar to other programming languages. Conditions have many form like the most basic form is `if` expression `then` statement where statement is only executed if expression is true.
|
|
|
|
```bash
|
|
if [expression]; then
|
|
will execute only if expression is true
|
|
else
|
|
will execute if expression is false
|
|
fi
|
|
```
|
|
|
|
Sometime if conditions becoming confusing so you can write the same condition using the `case statements`.
|
|
|
|
```bash
|
|
case expression in
|
|
pattern1 )
|
|
statements ;;
|
|
pattern2 )
|
|
statements ;;
|
|
...
|
|
esac
|
|
```
|
|
|
|
Expression Examples:
|
|
|
|
```bash
|
|
statement1 && statement2 # both statements are true
|
|
statement1 || statement2 # at least one of the statements is true
|
|
|
|
str1=str2 # str1 matches str2
|
|
str1!=str2 # str1 does not match str2
|
|
str1<str2 # str1 is less than str2
|
|
str1>str2 # str1 is greater than str2
|
|
-n str1 # str1 is not null (has length greater than 0)
|
|
-z str1 # str1 is null (has length 0)
|
|
|
|
-a file # file exists
|
|
-d file # file exists and is a directory
|
|
-e file # file exists; same -a
|
|
-f file # file exists and is a regular file (i.e., not a directory or other special type of file)
|
|
-r file # you have read permission
|
|
-s file # file exists and is not empty
|
|
-w file # your have write permission
|
|
-x file # you have execute permission on file, or directory search permission if it is a directory
|
|
-N file # file was modified since it was last read
|
|
-O file # you own file
|
|
-G file # file's group ID matches yours (or one of yours, if you are in multiple groups)
|
|
|
|
file1 -nt file2 # file1 is newer than file2
|
|
file1 -ot file2 # file1 is older than file2
|
|
|
|
-lt # less than
|
|
-le # less than or equal
|
|
-eq # equal
|
|
-ge # greater than or equal
|
|
-gt # greater than
|
|
-ne # not equal
|
|
```
|
|
|
|
## 2.6. Loops
|
|
|
|
There are three types of loops in bash. `for`, `while` and `until`.
|
|
|
|
Different `for` Syntax:
|
|
```bash
|
|
for x := 1 to 10 do
|
|
begin
|
|
statements
|
|
end
|
|
|
|
for name [in list]
|
|
do
|
|
statements that can use $name
|
|
done
|
|
|
|
for (( initialisation ; ending condition ; update ))
|
|
do
|
|
statements...
|
|
done
|
|
```
|
|
|
|
`while` Syntax:
|
|
```bash
|
|
while condition; do
|
|
statements
|
|
done
|
|
```
|
|
|
|
`until` Syntax:
|
|
```bash
|
|
until condition; do
|
|
statements
|
|
done
|
|
```
|
|
|
|
# 3. Tricks
|
|
|
|
## Set an alias
|
|
Open `bash_profile` by running following command `nano ~/.bash_profile`
|
|
> alias dockerlogin='ssh www-data@adnan.local -p2222' # add your alias in .bash_profile
|
|
|
|
## To quickly go to a specific directory
|
|
nano ~/.bashrc
|
|
> export hotellogs="/workspace/hotel-api/storage/logs"
|
|
|
|
```bash
|
|
source ~/.bashrc
|
|
cd $hotellogs
|
|
```
|
|
|
|
## Exit traps
|
|
|
|
Make your bash scripts more robust by reliably performing cleanup.
|
|
|
|
```bash
|
|
function finish {
|
|
# your cleanup here. e.g. kill any forked processes
|
|
jobs -p | xargs kill
|
|
}
|
|
trap finish EXIT
|
|
```
|
|
|
|
## Saving your environment variables
|
|
|
|
When you do `export FOO = BAR`, your variable is only exported in this current shell and all its children, to persist in the future you can simply append in your `~/.bash_profile` file the command to export your variable
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|
```bash
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echo export FOO=BAR >> ~/.bash_profile
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|
```
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|
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|
## Accessing your scripts
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|
|
|
You can easily access your scripts by creating a bin folder in your home with `mkdir ~/bin`, now all the scripts you put in this folder you can access in any directory.
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|
|
|
If you can not access, try append the code below in your `~/.bash_profile` file and after do `source ~/.bash_profile`.
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|
```bash
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|
# set PATH so it includes user's private bin if it exists
|
|
if [ -d "$HOME/bin" ] ; then
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|
PATH="$HOME/bin:$PATH"
|
|
fi
|
|
```
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|
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|
# 4. Debugging
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|
You can easily debug the bash script by passing different options to `bash` command. For example `-n` will not run commands and check for syntax errors only. `-v` echo commands before running them. `-x` echo commands after command-line processing.
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|
|
|
```bash
|
|
bash -n scriptname
|
|
bash -v scriptname
|
|
bash -x scriptname
|
|
```
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|
|
|
## Contribution
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|
|
|
- Report issues [How to](https://help.github.com/articles/creating-an-issue/)
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|
- Open pull request with improvements [How to](https://help.github.com/articles/about-pull-requests/)
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|
- Spread the word
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|
|
## License
|
|
|
|
[](https://creativecommons.org/licenses/by/4.0/)
|