Bash script advanced?

Wed Nov 26 2025 00:00:00 GMT+0000 (Coordinated Universal Time)

#!/bin/sh — comment that tells the shell which interpreter to use (“shebang”).

To source something in all new terminals:

nano ~/.bashrc`

# which shell I'm using
echo $SHELL

which bash

Then include your source command.

# create the script
nano myscript.sh

# enable execution permission — after this the file looks green
sudo chmod +x myscript.sh

# execute
./myscript.sh

Variables

# without space, assign variable; '' for var_name and "" for var_value
myname="vk"
echo $myname

# subshell
files=$(ls)
echo $files

# all caps → environmental var, all lower → local var
echo $USER

Math Functions

# evaluate expression
expr 10 + 30
expr 10 - 30
expr 10 / 30
expr 10 \* 30

if / else

[] is shorthand for the test command.

#if statements
if [ ]; then
else
fi

# comparison
!     # not
-ne   # not equal
-gt   # greater than

# check file existence
if [ -f ~/myfile ]; then
    echo "file exists"
else
    echo "file does not exist"
fi

Install a package if not present:

#!/bin/bash
command=/usr/bin/htop

if [ -f $command ]; then
    echo "$command is available, let's run it"
else
    echo "$command is not available, let's install"
    sudo apt update && sudo apt install -y htop
    # && executes second command only if first succeeds
fi

$command

Exit Codes

Check whether the previous command was successful:

echo $? — success → 0, failure → non-zero.

$? outside an if block may show 0 because echo itself succeeded. Always check $? immediately after the command you want to test.

exit 1 — force exit code to 1; commands after exit will not run.

Useful special variables:

$0 — invoked script name

$1 — first argument to the script

$# — number of arguments

$? — exit status of the most recent command

Example:

if [ $? -eq 0 ]; then
    echo "success"
    exit 0
else
    echo "failed: expected ${MATCHSTR} in ${OUTPUTSTRING} but instead found"
    exit 1
fi