Scripts

Published: 2023-12-11
Updated: 2024-12-22

Shebang Line 🔗

Source: https://codejunction.hashnode.dev/the-advantage-of-using-usrbinenv-bash-over-binbash

This is the line that goes at the top of the script and tells the shell what to use to execute the script

#!/usr/bin/env bash

If statements 🔗

Source: https://linuxize.com/post/bash-if-else-statement/

WARNING!! I ran into syntax errors with fi if I put then on the same line with if (see error message below)

… syntax error near unexpected token `fi’

if TEST-COMMAND1
then
  STATEMENTS1
elif TEST-COMMAND2
then
  STATEMENTS2
else
  STATEMENTS3
fi

#!/bin/bash

echo -n "Enter a number: "
read VAR

if [ $VAR -gt 10 ]
then
  echo "The variable is greater than 10."
elif [ $VAR -eq 10 ]
then
  echo "The variable is equal to 10."
else
  echo "The variable is less than 10."
fi

Until 🔗

#!/bin/bash

counter=0

until [ $counter -gt 5 ]
do
  echo Counter: $counter
  ((counter++))
done

Positional arguments 🔗

Source: https://www.redhat.com/en/blog/arguments-options-bash-scripts

$0 - the name of the running script and $1 to $9 are positional arguments. The example if for a executable text file called test_script in your current directory.

#!/usr/bin/env bash

echo Running \"$0\"
echo With 1st argument in quotes is \"$1\"
echo And 2nd without quotes this time is: "$2"
echo And 3rd without quotes this time is: $3

Run with ./test_script.sh first second third and it outputs

Running "./test_script.sh"
With 1st argument in quotes is "first"
And 2nd without quotes this time is: second
And 3rd without quotes this time is: third

Case statements 🔗

Source: https://linuxize.com/post/bash-case-statement/

case EXPRESSION in

  PATTERN_1)
    STATEMENTS
    ;;

  PATTERN_2)
    STATEMENTS
    ;;

  PATTERN_N)
    STATEMENTS
    ;;

  *)
    STATEMENTS
    ;;
esac

#!/bin/bash

echo -n "Enter the name of a country: "
read COUNTRY

echo -n "The official language of $COUNTRY is "

case $COUNTRY in

  Lithuania)
    echo -n "Lithuanian"
    ;;

  Romania | Moldova)
    echo -n "Romanian"
    ;;

  Italy | "San Marino" | Switzerland | "Vatican City")
    echo -n "Italian"
    ;;

  *)
    echo -n "unknown"
    ;;
esac

Comparing Strings 🔗

Source: https://linuxize.com/post/how-to-compare-strings-in-bash/

See link for more info. One note I wanted to add is to ensure you use quotes around your strings especially if they are coming from variables.

Directory exits test (and negation) 🔗

The example below does nothing in the positive case and exits on the negative case

if [ -d "$SRC_DIR" ] 
then
    : # Do nothing (was getting trouble with negating the condition)
else
  echo "Error: Source directory does not exist"
  exit 1
fi

No-Op 🔗

Source: https://stackoverflow.com/questions/12404661/what-is-the-use-case-of-noop-in-bash

Simply use a :

while keep_waiting; do
  : # do nothing
done

Set script to exit on errors 🔗

Source: https://www.gnu.org/software/bash/manual/html_node/The-Set-Builtin.html

set -e

Set script to echo commands 🔗

Source: https://stackoverflow.com/questions/2853803/how-to-echo-shell-commands-as-they-are-executed

set -x