bash

Bash script execution methods

Method	Makes script executable	Runs in current session
`source test.sh`	No	Yes
`. test.sh`	No	Yes
`./test.sh`	Yes	No
`bash ./test.sh`	No	No
`sh ./test.sh`	No	No

Capture or ignore the standard output (STDOUT) and standard error (STDERR) of your command

This section uses the following directory structure as an example:

tree .
.
├── bar
│   ├── test1
│   ├── test2
│   └── test3
└── foo
    ├── test1
    ├── test2
    └── test3

3 directories, 6 files

When you run ls bar foo test, it produces both output and error:

ls bar foo test
#ls: test: No such file or directory
#bar:
#test1 test2 test3
#
#foo:
#test1 test2 test3

To capture STDOUT to the output.txt file while displaying STDERR:

ls bar foo test > output.txt
#ls: test: No such file or directory

cat output.txt
#bar:
#test1
#test2
#test3
#
#foo:
#test1
#test2
#test3

To capture STDOUT to the output.txt file and capture STDERR to the error.txt file:

ls bar foo test > output.txt 2>error.txt

cat output.txt
#bar:
#test1
#test2
#test3
#
#foo:
#test1
#test2
#test3

cat error.txt
#ls: test: No such file or directory

To capture both STDOUT and STDERR to the log.txt file:

ls bar foo test > log.txt 2>&1

cat log.txt
#ls: test: No such file or directory
#bar:
#test1
#test2
#test3
#
#foo:
#test1
#test2
#test3

To append both STDOUT and STDERR to an existing log file:

echo "Logs:" > log.txt
ls bar foo test >> log.txt 2>&1

cat log.txt
#Logs:
#ls: test: No such file or directory
#bar:
#test1
#test2
#test3
#
#foo:
#test1
#test2
#test3

To ignore STDOUT:

ls bar foo test > /dev/null
#ls: test: No such file or directory

To ignore both STDOUT and STDERR:
```
ls bar foo test > /dev/null 2>&1
```
The following command might not work as expected:
```
ls bar foo test 2>&1 > /dev/null
#ls: test: No such file or directory
```
For more information, see Stackoverflow/16283739.

Control operators `&` and `&&`

When resolving cherry-pick conflicts on multiple branches (PRs), I often use the following command:

gh pr checkout PR_NUMBER
# Resolve conflicts
# Commit and push
git commit -a -S -m "resolve conflicts" && git push &
# Repeat for other PRs
gh pr checkout PR_NUMBER

The & operator is used to run the command in the background. The && operator is used to run the git push command only if the git commit command succeeds.

quote

If a command is terminated by the control operator &, the shell executes the command in the background in a subshell. The shell does not wait for the command to finish, and the return status is 0.

An AND list has the form

command1 && command2

command2 is executed if, and only if, command1 returns an exit status of zero.

For more details, refer to Shell Grammar: Lists.

tip

You can also use the bg command to run a command in the background. For example:

Run a time-consuming command in the foreground, such as git pull.
Press Ctrl+Z in the terminal to stop the git pull command. (Ctrl+Z sends the SIGTSTP signal to the process.)
Run the bg command to continue the git pull command in the background.

For more details, refer to Job Control Builtins.

History expansion

Bash provides a history expansion feature that enables you to recall or edit commands from the history list using event designators.

The following is an example:

$ cd Projects/docs-1
$ find . -name "TOC.md"
./TOC.md
$ cd ../docs-2

# Refer to the command 2 lines back.
$ !-2
find . -name "TOC.md"
./TOC.md
$ history | grep "find"
10000  find . -name "TOC.md"

# Refer to command line 10000.
$ !10000
find . -name "TOC.md"
./TOC.md

quote

Use touch to create a new file called semester in missing.
Write the following into that file, one line at a time:
```
#!/bin/sh
curl --head --silent https://missing.csail.mit.edu
```
The first line might be tricky to get working. It’s helpful to know that # starts a comment in Bash, and ! has a special meaning even within double-quoted (") strings. Bash treats single-quoted strings (') differently: they will do the trick in this case.

—— The Missing Semester of Your CS Education > Course overview + the shell > Exercises

Using double-quoted strings here returns an error:

echo "#!/bin/sh\ncurl --head --silent https://missing.csail.mit.edu" > semester
#sh: event not found: /bin/sh\ncurl

To complete this task, use the following command:

echo '#!/bin/sh\ncurl --head --silent https://missing.csail.mit.edu' > semester
cat semester

The output is as follows:

#!/bin/sh
curl --head --silent https://missing.csail.mit.edu

For more details, refer to History Expansion.

Remove a substring from a string

You can get more solutions from Stackoverflow/16623835.

For more details about #, ##, %, and %%, refer to Shell Parameter Expansion.

Remove the prefix using `#`

In ${parameter#word}, # is used to remove the shortest matching pattern word from the beginning of parameter.

var="test-123"
echo "${var#test-}" # 123

When you use find ${DIR_PATH} command to search for files, the result will be prefixed with ${DIR_PATH}. To remove the prefix, you can use the # operator. For example:

find ${DIR_PATH} | while IFS= read -r DIR; do
    echo "${DIR#${DIR_PATH}}"
done

You can also use the sed command to perform the same operation. For more details, refer to sed.

find ${DIR_PATH} | while IFS= read -r DIR; do
    echo "${DIR}" | sed "s~^${DIR_PATH}~~"
done

The following example shows the result before and after removing the prefix:

Before
After

find . -maxdepth 3 -mindepth 3 | while IFS= read -r DIR; do
    echo "${DIR}"
done
# ./website/docs
# ./website/blog
# ./website/yarn.lock
# ./website/package.json
# ./website/static
# ./website/docsearch.json

find . -maxdepth 3 -mindepth 3 | while IFS= read -r DIR; do
    echo "${DIR#./}"
done
# website/docs
# website/blog
# website/yarn.lock
# website/package.json
# website/static
# website/docsearch.json

tip

What is the difference between # and ##?

# removes the shortest matching pattern word from the beginning of parameter.
## removes the longest matching pattern word from the beginning of parameter.

var="test-test-123"
echo "${var#test-}" # test-123
echo "${var#*test-}" # test-123
echo "${var##*test-}" # 123

In the following example, a*b pattern matches any sequence that starts with a and ends with b.

var="aaabbbccc"
echo "${var#*a}" # aabbbccc
echo "${var##*a}" # bbbccc
echo "${var#a*b}" # bbccc
echo "${var##a*b}" # ccc
echo "${var#a*c}" # cc
echo "${var##a*c}" #

Remove the suffix using `%`

In ${parameter%word}, % is used to remove the shortest matching pattern word word from the end of parameter.

var="test-123"
echo "${var%-123}" # test

tip

What is the difference between % and %%?

% removes the shortest matching pattern word from the end of parameter.
%% removes the longest matching pattern word from the end of parameter.

Bash script execution methods​

Capture or ignore the standard output (STDOUT) and standard error (STDERR) of your command​

Control operators & and &&​

History expansion​

Remove a substring from a string​

Remove the prefix using #​

Remove the suffix using %​