Shell (Bash) Cheatsheet

Structures

if [[ ! -f .bashrc ]]; then
    echo "conf is not found"
    touch .bashrc
fi

is equivalent to:

[[ -f .bashrc]] || {
    echo "conf is not found"
    touch .bashrc
}

function

function f() {
    echo $1
    return 1
}
# Or:
f() {
    echo $1
    return 1
}

• $1, $2 etc is the 1st, 2nd etc argument
• $# is the amount of arguments
• $* / [email protected] is an array of all arguments
• return: built-in command for exiting function (with an exit code)

case

case $var in
    1)
        echo one;;
    2)
        echo two;;
    *)
        echo other;;
esac

for

for var in one two three
do
    echo $var
done

for (( i=0; i<10; i++ ))
do
    echo $i
done

if

if <list0>; then
    <list1>
else
    <list2>
fi

Run <list1> when <list0> exit with code 0, elsewhere run <list2>.

Brackets

[ ] / [[ ]]

(Built-in command) Conditional brackets, you can think the left bracket as a program. The “program” returns an error code depending on the <expression>. [[ is an enhanced version of [ which may not be available in lower versions of bash.

if [[ <expression> ]]; then
    <list>
fi

Here's an incomplete list of <expression>:

• -a <file>: true when file exist
• -v <var>: true when variable is set
• str1 <|>|==|=|!= str2: compare strings lexicographically
• str1 <OP> str2: compare numerically, where <OP> is:
  • -eq, -ne: equal to, not equal to
  • -lt, -le: less than, less than or equal to
  • -gt, -ge: greater than, greater than or equal to

(( ))

(Built-in command) Exit with code 0 when it's a non-zero number.

(( 1 )) # code 0
(( 0 )) # code 1
(( nan )) # code 1

$[ ]

(Keyword) Math brackets.

echo $[ 9 * 9 ] # 81

$( )

(Keyword) Equivalent to `` .

Variable

There are two types of variables, string and array.

# Create a string variable $var with value '233', note that there should be no space surrounding '='
var=233
# Get the value of $var
echo $var
# Create an array variable
arr=(first second third)

Tip: var=string with-space is invalid, it will execute command with-space with $var set to string, use quotes in these cases.

Built-in variables

String clipping

${<variable>:<start>:<length>}

str="Bourne-Again Shell's great"
echo ${str:7:5} # Again

String splitting

var="123/456/789"
echo ${var#*/}    # 456/489
echo ${var##*/}   # 789
echo ${var%/*}    # 123/456
echo ${var%%/*}   # 123

|->|
123/456/789
|----->|

       |<-|
123/456/789
   |<-----|

String substitution

Array

Quotes

• "": parse variables and escape characters in it
• '': what you see is what you get

echo ~    # /home/genel
echo "~"  # ~
echo *    # documents downloads pictures music
echo "*"  # *

Pipes

a << delimiter
    This is here-doc.
    You can write anything here
    as the stdin of 'a',
    until your 'delimiter' appears.
delimiter

In some cases (e.g. in scripts), you don't want program to print anything to the session. Redirect the stdout and stderr of the program to /dev/null.

Built-in command

Variable

Variables are global by default, use a local keyword to limit their scopes.

f() { a=2333; }
f
echo $a    # 2333
g() { local b=2333; }
g
echo $b    # ''

Process

In REPL, press Ctrl-Z to suspend a process.

kill is also an external program, but the built-in one has more capabilities. e.g. You can specify <process> not only with PID, but also with "%<process_name>".

Other commands

Others