bash¶

To implement a for loop

for WORD in LIST; do
  COMMANDS
done

For example

for CurDay in Monday Tuesday Wednesday Thursday Friday Saturday Sunday; do
  printf "%s\n" "$CurDay"
done

To implement a case statement

case $1 in
    0)
        echo "Found a '0'." ;;
    1)
        echo "Found a '1'." ;;
    2)
        echo "Found a '2'." ;;
    3*)
        echo "Something beginning with '3' found." ;;
    '')
        echo "Nothing (null) found." ;;
    *)
        echo "Anything else found." ;;
esac

Turn on built-in Bash debugging output

set -x

Turn the above off again

set +x

Retrieve N-th piped command exit status

printf 'foo' | grep -F 'foo' | sed 's/foo/bar/'
echo ${PIPESTATUS[0]}  # replace 0 with N

Lock file

( set -o noclobber; echo > my.lock ) || echo 'Failed to create lock file'

Fork bomb. Do not run this! Has the potential to wreak havoc. It repeatedly

and quickly spawns a lot of processes until the system eventually locks up.

:(){ :|:& };:

An alternative, easier-to-understand version without the obfuscation

func(){ func | func & }; func

Unix Roulette, courtesy of Bigown's answer in the joke thread

DANGER! Don't execute!

Luckily, most modern setups have --preserve-root on by default, so this will be null and void, but even so, not even remotely worth the risk!

[ $[ $RANDOM % 6 ] == 0 ] && rm -rf /* || echo Click #Roulette

A for loop one-liner

for CurIter in {1..4}; do echo "$CurIter"; done

Alternative, slightly-cleaner syntax

for CurIter in {1..4}; { echo "$CurIter"; }

Test for a variable being equal to (-eq) an integer (0)

if [ $var -eq 0 ]; then
  printf "Variable '\$var' is equal to '0'.\n"
fi

Test for a PATH executable existing as a file, but note that aliases and

functions will also output and result in a 0 exit status.

command -v ${program} >/dev/null 2>&1 || error "${program} not installed"

However, that is a solution commonly found in a script using the Bourne

shell, so in this case, an alternative, Bash-like, and more accurate version could be:

if ! type -fP bash > /dev/null 2>&1; then
  printf "ERROR: Dependency 'bash' not met." >&2
  exit 1
fi

Send both STDOUT and STDERR from COMMAND to FILE

COMMAND > FILE 2>&1

Send STDOUT and STDERR from COMMAND to /dev/null, where data goes to die

COMMAND > /dev/null 2>&1

Pipe the STDOUT and STDERR from COMMAND_1 to COMMAND_2

COMMAND_1 |& COMMAND_2

Verbosely convert whitespaces () to underscores (_) in file names

for name in *\ *; do mv -vn "$name" "${name// /_}"; done

Expand a regular variable

$Var

Some people like to cuddle the variable name with braces ('{' and '}') but

this is usually superfluous and wasted keystrokes, unless you need to protect the variable name from having other characters included, as in the below example, or you're using one of the many features of parameter expansion.

"${Var}some text"

Access a given index in an array. In this example, you don't technically

need to specify the element, because by default the first element is used. As with many other languages, note that indices are 0-first, so 1 is the 2^nd.

${Var[0]}

You can use arithmetic between '[' and ']' as well

${Var[2+3]}

Expand variable to the length of that to which the variable would expand

${#Var}

Expand array variable to the number of elements/indices. You may find that

[*] works as well as [@], in this case.

${#Var[@]}

Expand variable to a substring. In this case, imagine Var is equal to the

string 'thing', but the offset is 2 and the length is 1, giving us an 'i'.

${Var:2:1}

Expand variable to a substring by removing the matched glob pattern from left

to right. To make this global (IE: greedy) use two '#' characters. So, in this example, everything, from left to right, up to and including a 'T' or 't', will be removed, but it would only happen once.

"${Var#*t}"

As above, but from right to left. Use two '%' characters for a greedy match

"${Var%t*}"

Change how the variable expands by using pattern substitution. This uses glob

pattern matching, not REGEX. If the first '/' is instead '//', a greedy match is performed.

${Var/PATTERN/REPLACEMENT/}

A good example of the above, which will list directories in PATH which exist

and are directories. It works because all instances of ':' are replaced with a whitespace, causing find(1) to see multiple directories (fields) in which to search.

find ${PATH//:/ } -type d

Expand the variable to the string between :- and the closing }, if that

variable doesn't already have a value. This can be useful to set a default.

${Var:-Default Value}

This is a way of displaying an error message if the contents of the variable

is empty. It will also immediately exit with an exit status of 1.

${Var:?This is an error message.}

Indirect expansion exists in a couple of ways in BASH. If Var is equal to

OtherVar, and that OtherVar is equal to true, the below example would expand to true.

${!Var}

Expand variable so that the first letter is uppercase. Use two '^' (carets)

if you want the entire contents of the variable to change to uppercase.

${Var^}

As above, but convert to lowercase. Use two ',' characters to transform the

entire string to which Var expands.

${Var,}

cheat:bash

To implement a for loop:

for file in *; do 
  echo $file found;
done

To implement a case command

case "$1"
in
    0) echo "zero found";;
    1) echo "one found";;
    2) echo "two found";;
    3*) echo "something beginning with 3 found";;
esac

To turn on debugging

set -x

To turn off debugging

set +x

Retrieve N-th piped command exit status

printf 'foo' | fgrep 'foo' | sed 's/foo/bar/'
echo ${PIPESTATUS[0]}  # replace 0 with N

To create a lockfile

( set -o noclobber; echo > my.lock ) || echo 'Failed to create lock file'

tldr:bash

bash Bourne-Again SHell, an sh-compatible command-line interpreter. See also: zsh, histexpand (history expansion). More information: https://www.gnu.org/software/bash/. Start an interactive shell session:

bash

Start an interactive shell session without loading startup configs

bash --norc

Execute specific [c]ommands

bash -c "echo 'bash is executed'"

Execute a specific script

bash path/to/script.sh

Execute a specific script while printing each command before executing it

bash -x path/to/script.sh

Execute a specific script and stop at the first [e]rror

bash -e path/to/script.sh

Execute specific commands from stdin

echo "echo 'bash is executed'" | bash

Execute remote script

bash -c "$(curl -fsSL https://example.com/script.sh)"

Get directory path

DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"