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Introduction to Shell/Bash Scripts

A shell script is a computer program designed to be run by the Unix shell, a command-line interpreter. The various dialects of shell scripts are considered to be scripting languages. Typical operations performed by shell scripts include file manipulation, program execution, and printing text. A script which sets up the environment, runs the program, and does any necessary cleanup, logging, etc. is called a wrapper.

#!/bin/sh
echo "What is your name?"
read USER_NAME
echo "Hello $USER_NAME"
echo "I will create you a file called $USER_NAME_file"
touch $USER_NAME_file

Passing Variables​

#!/bin/bash
var_name=$1
echo ${var_name}
echo $2

./test hello 2

Shortcuts​

$0 - Script name
$# - Argument count
$? - Most recent exit code
$@ and $* - All the args

https://dev.to/rpalo/handling-arguments-in-bash-scripts-3o5m

Loops​

#!/bin/sh
for i in 1 2 3 4 5
do
  echo "Looping ... number $i"
done

 for i in {1..10}
 do
  echo "$i"
 done

 #!/bin/sh
for i in hello 1 * 2 goodbye
do
  echo "Looping ... i is set to $i"
done

 #!/bin/sh
INPUT_STRING=hello
while [ "$INPUT_STRING" != "bye" ]
do
  echo "Please type something in (bye to quit)"
  read INPUT_STRING
  echo "You typed: $INPUT_STRING"
done

#!/bin/zsh
# check if server name is passed
if [ "$1" ]; then
        # server name is passed, ssh
        ssh ubuntu@$1.abc.com
else
        # get server name since not passed
        echo "Enter server name to ssh - "
        read SERVER_NAME
        ssh ubuntu@$SERVER_NAME.abc.com
fi

Examples​

# Write date to a file every second
#!/bin/bash
mkdir -p /var/log/apache
touch /var/log/apache/access.log
while true
do
        echo $(date) >> /var/log/apache/access.log
    sleep 1
done

# Loop through and run all commands
#!/bin/bash

VAL="$(kubectl get pods -n example | awk '{if (NR!=1) { print $1 }}')"

for i in $VAL
do
        kubectl delete -n example pods $i
done

while true; do echo `date` | kafkacat -P -b my-cluster-kafka-brokers.kafka:9092 -t test; done

# find and replace filename
find . -name '*---*' -type f | grep -v "media" -exec sh -c '
for f; do
    mv "$f" "${f%/*}/${f##*/123_}"
done' sh {} +

find . -type f | grep -v "media" | sed 's|\(.*/\)[^A-Z]*\([A-Z].*\)|mv \"&\" \"\1\2\"|' | sh

Others​

Shell Scripting: Error Handling with set -e​

By default, Bash scripts continue executing even if a command fails (returns a non-zero exit code). Using set -e (also known as errexit) changes this behavior to make scripts more robust.

Include set -e at the top of your scripts to ensure they exit immediately upon the first failure:

Why use it?​

  • Prevents Domino Effects: Stops a script before it can run "downstream" logic on bad data or in the wrong directory.
  • Easier Debugging: The script dies exactly where the problem occurred, rather than failing silently and causing issues later.

Exceptions (When it won't exit)​

The shell will not exit if the failing command is:

  1. Part of an if or while test.
  2. Part of a logical list (e.g., command || echo "failed").
  3. Preceded by ! (inverted exit code).

Best Practice: The "Unofficial Bash Strict Mode"​

For maximum reliability, many developers use this combined string:

set -euo pipefail
  • -e: Exit on error.
  • -u: Exit if an undefined variable is used (prevents typos).
  • -o pipefail: Returns the exit code of the first command in a pipeline that fails (e.g., grep "x" | sort will fail if grep fails).

References​