UNIX shell scripting: frequently asked questions

General

List environment variables in the current shell session

To list all environment variables (parameters) currently defined in a shell session, run the command env.

Note that env only lists exported variables, i.e. variables that are available to subprocesses. To list all (exported and non-exported) variables, use set bash keyword.

Stop a task running in the background

Once a process is in the background, it can be brought back to the foreground with the command fg. Once in the foreground, the process can be aborted with "Ctrl + C".

Example:

sleep 30 &   # Start a process and send it to the background.
jobs         # displays all jobs currently running the background.
fg           # Bring the process to the foreground.

# At this point the process can be aborted with "Ctrl + C".

✨ Notes:

• If multiple processes are running in the background, fg %i can bring back the desired process. E.g. fg %3 will bring back the 3rd process to the foreground.
• Use jobs to show the list of jobs running in the background.

Alternatively, a process (in the background or not) can also be force-terminated with the kill <PID> command, where <PID> is the process ID.

Example:

kill 50548   # The number is the PID of the process.

Testing for file existence: why use [[ -f $name ]] rather than ls $name

Using ls prints the file name (if it exists) or an error message (if it does not), which in a scripting context is not something we want. It’s possible to re-direct the output of ls $file > /dev/null to avoid this, but it is less convenient.

About sub-shells

A sub-shell is a child shell process spawned by the current shell. It runs in a separate execution environment, allowing isolated command execution.

Example: change directory only inside the sub-shell. The current shell will remain in the same directory.

(cd /tmp && ls)  

• Sub-shells are crated by adding brackets () around an expression.
• Sub-shells inherit environment variables that are exported in their parent shell. Changes made in the sub-shell environment do not affect the parent shell.
• Command substitution $(command) are run in a sub-shell.

Re-running the previous command

• Executing !! in the shell will re-run the last command.
• With !!:gs/old/new we can re-run the last command, while replacing all instances of old with new.

Example:

echo "This is a test-1"
!!                       # will print -> This is a test-1
!!:gs/1/2                # will print -> This is a test-2

zsh vs. bash - some differences

No word splitting no globbing after variable expansion

my_var=${foo}

my_var=$~{foo}    # perform globbing after variable expansion.
my_var=$={foo}    # perform word splitting after variable expansion.
my_var=$~={foo}   # perform both globbing and word splitting.

Non-terminated quote with pattern: !"

From https://unix.stackexchange.com/questions/497328/zsh-thinks-unterminated-quote-if-preceded-by-exclamation-mark

If the shell encounters the character sequence !" in the input, the history mechanism is temporarily disabled until the current list is fully parsed. The !" is removed from the input, and any subsequent ! characters have no special significance.

see also man zshexpn(1) for more details.

MacOS troubleshooting

MacOS (zsh) error - date command does not have the -I option

On Linux systems, the date command generally has the -I option to format the output of the date command in ISO 8601 format:

date -I   # -> 2022-05-25

On Mac OS, the date command does not have this option. Instead, one must manually provide a format string:

date "+%Y-%m-%d"   # -> 2022-05-25

Windows troubleshooting

Windows: disabling EOL (end-of-line) automatic conversion by Git

Inside of the course's Git repo, type the following command: git config core.autocrlf false

If needed, reset the current content of the repo to the original version: git reset --hard HEAD

git config core.autocrlf false
git reset --hard HEAD