194 lines
2.9 KiB
Plaintext
194 lines
2.9 KiB
Plaintext
# FILE HANDLING
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**CAVEAT:** `bash` does not handle binary data properly in versions `< 4.4`.
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## Read a file to a string
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Alternative to the `cat` command.
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```shell
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file_data="$(<"file")"
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```
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## Read a file to an array (*by line*)
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Alternative to the `cat` command.
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```shell
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# Bash <4 (discarding empty lines).
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IFS=$'\n' read -d "" -ra file_data < "file"
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# Bash <4 (preserving empty lines).
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while read -r line; do
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file_data+=("$line")
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done < "file"
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# Bash 4+
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mapfile -t file_data < "file"
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```
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## Get the first N lines of a file
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Alternative to the `head` command.
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**CAVEAT:** Requires `bash` 4+
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**Example Function:**
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```sh
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head() {
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# Usage: head "n" "file"
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mapfile -tn "$1" line < "$2"
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printf '%s\n' "${line[@]}"
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}
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```
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**Example Usage:**
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```shell
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$ head 2 ~/.bashrc
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# Prompt
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PS1='➜ '
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$ head 1 ~/.bashrc
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# Prompt
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```
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## Get the last N lines of a file
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Alternative to the `tail` command.
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**CAVEAT:** Requires `bash` 4+
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**Example Function:**
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```sh
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tail() {
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# Usage: tail "n" "file"
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mapfile -tn 0 line < "$2"
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printf '%s\n' "${line[@]: -$1}"
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}
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```
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**Example Usage:**
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```shell
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$ tail 2 ~/.bashrc
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# Enable tmux.
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# [[ -z "$TMUX" ]] && exec tmux
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$ tail 1 ~/.bashrc
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# [[ -z "$TMUX" ]] && exec tmux
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```
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## Get the number of lines in a file
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Alternative to `wc -l`.
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**Example Function (bash 4):**
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```sh
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lines() {
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# Usage: lines "file"
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mapfile -tn 0 lines < "$1"
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printf '%s\n' "${#lines[@]}"
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}
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```
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**Example Function (bash 3):**
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This method uses less memory than the `mapfile` method and works in `bash` 3 but it is slower for bigger files.
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```sh
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lines_loop() {
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# Usage: lines_loop "file"
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count=0
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while IFS= read -r _; do
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((count++))
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done < "$1"
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printf '%s\n' "$count"
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}
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```
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**Example Usage:**
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```shell
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$ lines ~/.bashrc
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48
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$ lines_loop ~/.bashrc
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48
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```
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## Count files or directories in directory
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This works by passing the output of the glob to the function and then counting the number of arguments.
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**Example Function:**
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```sh
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count() {
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# Usage: count /path/to/dir/*
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# count /path/to/dir/*/
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printf '%s\n' "$#"
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}
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```
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**Example Usage:**
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```shell
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# Count all files in dir.
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$ count ~/Downloads/*
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232
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# Count all dirs in dir.
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$ count ~/Downloads/*/
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45
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# Count all jpg files in dir.
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$ count ~/Pictures/*.jpg
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64
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```
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## Create an empty file
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Alternative to `touch`.
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```shell
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# Shortest.
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>file
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# Longer alternatives:
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:>file
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echo -n >file
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printf '' >file
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```
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## Extract lines between two markers
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**Example Function:**
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```sh
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extract() {
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# Usage: extract file "opening marker" "closing marker"
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while IFS=$'\n' read -r line; do
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[[ $extract && $line != "$3" ]] &&
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printf '%s\n' "$line"
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[[ $line == "$2" ]] && extract=1
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[[ $line == "$3" ]] && extract=
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done < "$1"
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}
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```
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**Example Usage:**
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```shell
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# Extract code blocks from MarkDown file.
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$ extract ~/projects/pure-bash/README.md '```sh' '```'
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# Output here...
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```
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<!-- CHAPTER END -->
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