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Bash (Unix shell): Difference between revisions

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echo $myvariable $anothervar
echo $myvariable $anothervar
</syntaxhighlight>
</syntaxhighlight>
You can check your bash scripts using [https://www.shellcheck.net/ https://www.shellcheck.net/].


==Usage==
==Usage==
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[https://linuxize.com/post/bash-if-else-statement/ https://linuxize.com/post/bash-if-else-statement/]
[https://linuxize.com/post/bash-if-else-statement/ https://linuxize.com/post/bash-if-else-statement/]
<syntaxhighlight lang="bash">
<syntaxhighlight lang="bash">
if [[ $VAR -gt 10 ]] then
if [[ $VAR -gt 10 ]]; then
   echo "The variable is greater than 10."
   echo "The variable is greater than 10."
elif [[ $VAR -eq 10 ]] then
elif [[ $VAR -eq 10 ]]; then
   echo "The variable is equal to 10."
   echo "The variable is equal to 10."
else
else
   echo "The variable is less than 10."
   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 $STR ]
</syntaxhighlight>
<syntaxhighlight lang="bash">
# Check if user is root (https://stackoverflow.com/questions/18215973/how-to-check-if-running-as-root-in-a-bash-script)
if [ "$EUID" -ne 0 ]; then
  echo "Please run as root"
  exit
fi
fi
</syntaxhighlight>
</syntaxhighlight>
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* <code>-gt</code> is <code>></code>
* <code>-gt</code> is <code>></code>
* <code>-ge</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>==
==<code>.bashrc</code>==
Your <code>.bashrc</code> file will be loaded for each terminal.<br>
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">
<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>
</syntaxhighlight>
[[Category:Programming languages]]
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