Add log_all_output

README.md

@@ -1031,6 +1031,32 @@ echo -n >file
 printf '' >file
 ```
 
+## Use `exec` to change the shell's file descriptors.
+
+Calling `exec` with redirection to a file can be used for logging all output.
+
+**Example Function:**
+
+```sh
+log_all_output() {
+    # Usage: log_all_output log_filename
+    exec > "$1" 2>&1
+}
+```
+
+**Example Usage:**
+
+```shell
+echo This is not logged
+echo Nor is this error >&2
+
+# all output will go to this file
+log_all_output myoutput.log
+
+echo normal output
+echo this is an error >&2
+```
+
 ## Extract lines between two markers
 
 **Example Function:**

manuscript/chapter4.txt

@@ -164,6 +164,32 @@ echo -n >file
 printf '' >file
 ```
 
+## Use `exec` to change the shell's file descriptors.
+
+Calling `exec` with redirection to a file can be used for logging all output.
+
+**Example Function:**
+
+```sh
+log_all_output() {
+    # Usage: log_all_output log_filename
+    exec > "$1" 2>&1
+}
+```
+
+**Example Usage:**
+
+```shell
+echo This is not logged
+echo Nor is this error >&2
+
+# all output will go to this file
+log_all_output myoutput.log
+
+echo normal output
+echo this is an error >&2
+```
+
 ## Extract lines between two markers
 
 **Example Function:**

test.sh

@@ -205,6 +205,16 @@ test_split() {
     assert_equals "${result[*]}" "hello world my name is john"
 }
 
+test_log_all_output() {
+    (
+	log_all_output test.log
+	echo xx stdout
+	echo yy stderr >&2
+    )
+    contents="$(<"test.log")"
+    assert_equals "$contents" $'xx stdout\nyy stderr'
+}
+
 assert_equals() {
     if [[ "$1" == "$2" ]]; then
         ((pass+=1))
@@ -219,7 +229,7 @@ assert_equals() {
 }
 
 main() {
-    trap 'rm readme_code test_file' EXIT
+    trap 'rm readme_code test_file test.log' EXIT
 
     # Extract code blocks from the README.
     while IFS=$'\n' read -r line; do