Bash (Unix shell): Difference between revisions

Latest revision as of 00:44, 26 December 2023

Bash Scripting

Getting Started

Here is an example bash script

#!/bin/bash
# A simple variable example
myvariable=Hello
anothervar=Fred

echo $myvariable $anothervar

You can check your bash scripts using https://www.shellcheck.net/.

Usage

Basic If Statements

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

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

bash if statements

Some common expressions

# File exists and is a regular file.
[ -f $FILE ]
# File is non-zero size
[ -s $FILE ]
# String has length zero
[ -z $STR ]
# String has length non-zero
[ -n $STR ]

# Check if user is root (https://stackoverflow.com/questions/18215973/how-to-check-if-running-as-root-in-a-bash-script)
if [ "$EUID" -ne 0 ]; then
  echo "Please run as root"
  exit
fi

Comparisons

-eq is ==
-gt is >
-ge is >=

Arrays

DRIVES=(
  /media/veracrypt1
  /media/veracrypt2
  /media/veracrypt3
  /media/veracrypt4
  /media/veracrypt5
  /media/veracrypt6
  /media/veracrypt7
  /media/veracrypt8
)
# Add to the array
DRIVES+=(/media/veracrypt9)

for i in "${DRIVES[@]}"
do
        ls $i
done

Notes

"${DRIVES[@]}" means every element will be a new word
"${DRIVES[*]}" will expand the array into a single word

Functions

#!/bin/bash 
function say_hello {
    echo Hello $1
}
say_hello World

Redirects

Redirections

# `ls` is an example command

# Input to stdin
ls < word

# Output stdout to file
ls > file.txt

# Append stdout to file
ls >> file.txt

# Output stderr and stdout to file
ls > file.txt 2>&1
ls &> file.txt # Only works in bash, not sh

Trap

To make a bash script exit on ctrl c:

trap "exit" INT

trap 'kill $(jobs -p)' EXIT

Wait

Add wait at the end of a script to wait for all subprocesses.

Brace Expansion

When you type the following:

echo {red,green,blue}_apple

bash will print out

red_apple green_apple blue_apple

If you want to save this into a variable, you can save it as an array:

APPLES=({red,green,blue}_apple)
echo "${APPLES[@]}"

`--`

Reference

-- explicitly ends non-positional arguments for many commands

alias

You can add aliases to your .bashrc to make commmon commands shorter.

alias kc=kubectl
alias tb="tensorboard --logdir"

dirname

Stack overflow

How to get the source directory of the bash script

#!/bin/bash
DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" >/dev/null 2>&1 && pwd)"
cd "${DIR}" || exit 1

Check if running as sudo

Reference

if [ $EUID -ne 0 ]
  then echo "Please run as root"
  exit
fi

`.bashrc`

Your .bashrc file will be loaded for each terminal.

Presentation of Shell Variable (PS1)

Add the following to show your current working directory.

PS1='\[\e]0;\u@\h: \w\a\]${debian_chroot:+($debian_chroot)}\[\033[01;32m\]\u@\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\n\$ '

@@ Line 12: / Line 12: @@
 echo $myvariable $anothervar
 </syntaxhighlight>
+You can check your bash scripts using [https://www.shellcheck.net/ https://www.shellcheck.net/].
 ==Usage==
 ===Basic If Statements===
+[https://linuxize.com/post/bash-if-else-statement/ https://linuxize.com/post/bash-if-else-statement/]
+<syntaxhighlight lang="bash">
+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
+</syntaxhighlight>
+[https://tldp.org/LDP/Bash-Beginners-Guide/html/sect_07_01.html bash if statements]
+;Some common expressions:
+<syntaxhighlight lang="bash">
+# File exists and is a regular file.
+[ -f $FILE ]
+# File is non-zero size
+[ -s $FILE ]
+# String has length zero
+[ -z $STR ]
+# String has length non-zero
+[ -n $STR ]
+</syntaxhighlight>
-===Comparisons
+<syntaxhighlight lang="bash">
+# Check if user is root (https://stackoverflow.com/questions/18215973/how-to-check-if-running-as-root-in-a-bash-script)
+if [ "$EUID" -ne 0 ]; then
+  echo "Please run as root"
+  exit
+fi
+</syntaxhighlight>
+===Comparisons===
 * <code>-eq</code> is <code>==</code>
 * <code>-gt</code> is <code>></code>
 * <code>-ge</code> is <code>>=</code>
+===Arrays===
+<syntaxhighlight lang="bash">
+DRIVES=(
+  /media/veracrypt1
+  /media/veracrypt2
+  /media/veracrypt3
+  /media/veracrypt4
+  /media/veracrypt5
+  /media/veracrypt6
+  /media/veracrypt7
+  /media/veracrypt8
+)
+# Add to the array
+DRIVES+=(/media/veracrypt9)
+for i in "${DRIVES[@]}"
+do
+        ls $i
+done
+</syntaxhighlight>
+;Notes
+* <code>"${DRIVES[@]}"</code> means every element will be a new word
+* <code>"${DRIVES[*]}"</code> will expand the array into a single word
+===Functions===
+<syntaxhighlight lang="bash">
+#!/bin/bash
+function say_hello {
+    echo Hello $1
+}
+say_hello World
+</syntaxhighlight>
+===Redirects===
+[https://www.gnu.org/software/bash/manual/html_node/Redirections.html Redirections]
+<syntaxhighlight lang="bash">
+# `ls` is an example command
+# Input to stdin
+ls < word
+# Output stdout to file
+ls > file.txt
+# Append stdout to file
+ls >> file.txt
+# Output stderr and stdout to file
+ls > file.txt 2>&1
+ls &> file.txt # Only works in bash, not sh
+</syntaxhighlight>
+===Trap===
+To make a bash script exit on ctrl c:
+<syntaxhighlight lang="bash">
+trap "exit" INT
+</syntaxhighlight>
+<syntaxhighlight lang="bash">
+trap 'kill $(jobs -p)' EXIT
+</syntaxhighlight>
+===Wait===
+Add <code>wait</code> at the end of a script to wait for all subprocesses.
+===Brace Expansion===
+When you type the following:
+<syntaxhighlight lang="bash">
+echo {red,green,blue}_apple
+</syntaxhighlight>
+bash will print out
+<pre>
+red_apple green_apple blue_apple
+</pre>
+If you want to save this into a variable, you can save it as an array:
+<syntaxhighlight lang="bash">
+APPLES=({red,green,blue}_apple)
+echo "${APPLES[@]}"
+</syntaxhighlight>
+===<code>--</code>===
+[https://unix.stackexchange.com/questions/11376/what-does-double-dash-mean Reference]
+<code>--</code> explicitly ends non-positional arguments for many commands
+===alias===
+You can add aliases to your <code>.bashrc</code> to make commmon commands shorter.
+<syntaxhighlight lang="bash">
+alias kc=kubectl
+alias tb="tensorboard --logdir"
+</syntaxhighlight>
+==dirname==
+[https://stackoverflow.com/questions/59895/how-to-get-the-source-directory-of-a-bash-script-from-within-the-script-itself Stack overflow]
+How to get the source directory of the bash script
+<syntaxhighlight lang="bash">
+#!/bin/bash
+DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" >/dev/null 2>&1 && pwd)"
+cd "${DIR}" || exit 1
+</syntaxhighlight>
+==Check if running as sudo==
+[https://stackoverflow.com/questions/18215973/how-to-check-if-running-as-root-in-a-bash-script Reference]
+<syntaxhighlight lang="bash">
+if [ $EUID -ne 0 ]
+  then echo "Please run as root"
+  exit
+fi
+</syntaxhighlight>
 ==<code>.bashrc</code>==
 Your <code>.bashrc</code> file will be loaded for each terminal.<br>
-==Presentation of Shell Variable==
+===Presentation of Shell Variable (PS1)===
-Add the following to show your
+Add the following to show your current working directory.
 <syntaxhighlight lang="bash">
-PS1='\[\e]0;\u@\h: \w\a\]${debian_chroot:+($debian_chroot)}\[\033[01;32m\]\u@\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\$'
+PS1='\[\e]0;\u@\h: \w\a\]${debian_chroot:+($debian_chroot)}\[\033[01;32m\]\u@\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\n\$ '
 </syntaxhighlight>
+[[Category:Programming languages]]