Add log_all_output

README.md

@@ -1031,6 +1031,48 @@ echo -n >file
 printf '' >file
 ```
 
+## Use `exec` to redirect script output to different file descriptors
+
+`exec` can be used to redirect script output to file descriptors. In the example below, `stdout` and `stderr` are redirected to `myoutput.log` (a file).
+
+**Example Function:**
+
+```sh
+log_all_output() {
+    # Usage: log_all_output log_filename
+	exec 6>&1         # save stdout
+	exec 7>&2         # save stderr
+    exec &>"$1"
+}
+
+stop_logging_output() {
+	exec 1>&6 6>&-   # Restore stdout
+	exec 2>&7 7>&-   # Restore stderr
+}
+```
+
+**Example Usage:**
+
+```shell
+# These are *not* logged.
+printf 'This is a normal message.\n'
+printf 'This is a normal error message.\n' >&2
+
+# All script output will go to 'myoutput.log'.
+log_all_output myoutput.log
+
+# These are now logged.
+printf 'This is a logged message.\n'
+printf 'This is a logged  error message.\n' >&2
+
+# Return to normal.
+stop_logging_output
+
+# These are not logged.
+printf 'This is a normal message.\n'
+printf 'This is a normal error message.\n' >&2
+```
+
 ## Extract lines between two markers
 
 **Example Function:**

manuscript/chapter4.txt

@@ -164,6 +164,48 @@ echo -n >file
 printf '' >file
 ```
 
+## Use `exec` to redirect script output to different file descriptors
+
+`exec` can be used to redirect script output to file descriptors. In the example below, `stdout` and `stderr` are redirected to `myoutput.log` (a file).
+
+**Example Function:**
+
+```sh
+log_all_output() {
+    # Usage: log_all_output log_filename
+	exec 6>&1         # save stdout
+	exec 7>&2         # save stderr
+    exec &>"$1"
+}
+
+stop_logging_output() {
+	exec 1>&6 6>&-   # Restore stdout
+	exec 2>&7 7>&-   # Restore stderr
+}
+```
+
+**Example Usage:**
+
+```shell
+# These are *not* logged.
+printf 'This is a normal message.\n'
+printf 'This is a normal error message.\n' >&2
+
+# All script output will go to 'myoutput.log'.
+log_all_output myoutput.log
+
+# These are now logged.
+printf 'This is a logged message.\n'
+printf 'This is a logged  error message.\n' >&2
+
+# Return to normal.
+stop_logging_output
+
+# These are not logged.
+printf 'This is a normal message.\n'
+printf 'This is an normal error message.\n' >&2
+```
+
 ## Extract lines between two markers
 
 **Example Function:**

test.sh

@@ -205,6 +205,19 @@ test_split() {
     assert_equals "${result[*]}" "hello world my name is john"
 }
 
+test_log_all_output() {
+    (
+	log_all_output test.log
+	printf stdout
+	printf stderr >&2
+	stop_logging_output
+	printf stdout
+	printf stderr >&2
+    ) &> /dev/null
+    contents="$(<test.log)"
+    assert_equals "$contents" "stdoutstderr"
+}
+
 assert_equals() {
     if [[ "$1" == "$2" ]]; then
         ((pass+=1))
@@ -219,7 +232,7 @@ 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