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Bash

You should try to stay in bash as long as possible without dropping into a subshell (ie running another program).

Go immediately to $EDITOR

ctrl-x, e

Most commonly used commands

history | awk '{a[$2]++}END{for(i in a){print a[i] " " i}}' | sort -rn | head

Delete files that are not extension

rm !(*.foo|*.bar|*.baz)

Find, delete empty directories

find . -type d -empty -exec rmdir {} \;

Show your shell from a port

script -qf | tee >(nc -kl 5000) >(nc -kl 5001) >(nc -kl 5002)

Replace filename spaces with underscores

# util-linux-ng
rename " " _ *

Search for Unicode use in a tree

for FILE in $(find . -type f) ; do echo File: ${FILE}; perl -ane '{ if(m/[[:^ascii:]]/) {print  } } ' ${FILE}; done

"Press any key to continue"

read -sn 1 -p "Press any key to continue..."

Arrays

code desc
arr=() Create an empty array
arr=(1 2 3) Initialize array
${arr[N]} Retrieve Nth element
${arr[@]} Retrieve all elements
${!arr[@]} Retrieve array indices
${#arr[@]} Calculate array size
arr[N]=foo Overwrite Nth element
arr+=(bar) Append value 'bar'
arr=( $(cmd) ) Save cmd output as an array
unset arr[N] Delete element at N
${arr[@]:x:n} Retrieve n elements starting at index x
read -a arr Read STDIN as an array to arr variable

Conditional Expressions

code desc
-a file file exists
-d file file exists and is a directory
-h file file exists and is a symbolic link
-r file file exists and is readable
-w file file exists and is writeable
-x file file exists and is executable
-S file file exists and is a socket
-n string true if length of string is non-zero
-z string true if length of string is zero

Process Substitution

A temporary named pipe

diff <(grep lines file1) <(grep lines file2)
thing --output >(gzip > output.txt.gz)

Syntax cheatsheet

fun () { echo "totes a function"; exit 1 ; } #Don't forget trailing colon if one line

if [ -e file ] ; then
    echo "file exists"
fi

case expression in
       pattern1 )
        statements ;;
       pattern2 )
        statements ;;
       ...
esac

# Bash 4+
# for VARIABLE in 1 2 3 4 5
for VARIABLE in {1..5}
do
        command1
        command2
        commandN
done
# to do stepping, use {1..99..2}

set

[Bash manual] set builtin

flag desc
-e exit if a pipeline returns non-zero
-o pipefail return value of a pipeline is the value of the last (rightmost) command to exit with a non-zero status
-o posix match POSIX standard behaviour (https://www.gnu.org/software/bash/manual/html_node/Bash-POSIX-Mode.html#Bash-POSIX-Mode)
-n read commands but do not execute (used for checking syntax)
-u treat unset variables and parameters as an error when performing parameter expansion
-x print trace of commands as they are executed
-C prevent output redirection using '>', '>&', and '<>' from overwriting existing files

Using regex for variable testing

if [[ $HOSTNAME =~ host[0-9].example.com ]]; then
    echo "yay"
fi

Temporary directory/file

mktemp -d

Variables / functions

# Set an environment variable
declare -x BLARG=5
# Show the functions declared in the shell
declare -F
# show functions on ancient shells
typeset -F

Use heredocs

cat <<EOM > file.out
blah
blah
EOM

Quit without saving history

unset HISFILE && exit

Regex change over files returned from grep

# macOS
grep -l ... | xargs -I% sed -i".bkp" -e "s/old/new/" %

I/O Redirection

code description
&> Redirect both stderr and stdout
1> Redirect stdout
2> Redirect stderr
2>&1 Redirect stderr to stdout
< Redirect stdin to a process