Bash scripting cheatsheet
This is a quick reference to getting started with Bash scripting.
Example
#!/usr/bin/env bash name="John" echo "Hello $name!"
Variables
name="John" echo $name # see below echo "$name" echo "$!"
Generally quote your variables unless they contain wildcards to expand or command fragments.
wildcard="*.txt" options="iv" cp -$options $wildcard /tmp
String quotes
name="John" echo "Hi $name" #=> Hi John echo 'Hi $name' #=> Hi $name
Shell execution
echo "I'm in $(pwd)" echo "I'm in `pwd`" # obsolescent # Same
Conditional execution
git commit && git push git commit || echo "Commit failed"
Functions
get_name() < echo "John" >echo "You are $(get_name)"
Conditionals
if [[ -z "$string" ]]; then echo "String is empty" elif [[ -n "$string" ]]; then echo "String is not empty" fi
Strict mode
Brace expansion
Parameter expansions
Basics
name="John" echo "$" echo "$" #=> "john" (substitution) echo "$" #=> "Jo" (slicing) echo "$" #=> "Jo" (slicing) echo "$" #=> "Joh" (slicing) echo "$" #=> "n" (slicing from right) echo "$" #=> "h" (slicing from right) echo "$" #=> $food or "Cake"
str="/path/to/foo.cpp" echo "$" # /path/to/foo echo "$.o" # /path/to/foo.o echo "$" # /path/to echo "$" # cpp (extension) echo "$" # foo.cpp (basepath) echo "$" # path/to/foo.cpp echo "$" # foo.cpp echo "$" # /path/to/bar.cpp
str="Hello world" echo "$" # "world" echo "$" # "world"
src="https://devhints.io/path/to/foo.cpp" base=$ #=> "foo.cpp" (basepath) dir=$ #=> "/path/to/" (dirpath)
Substitution
| Code | Description |
| $ | Remove suffix |
| $ | Remove prefix |
| $ | Remove long suffix |
| $ | Remove long suffix |
| $ | Remove long prefix |
| $ | Remove long prefix |
| $ | Replace first match |
| $ | Replace all |
| $ | Replace suffix |
| $ | Replace prefix |
: ' This is a multi line comment '
Substrings
Length
Manipulation
str="HELLO WORLD!" echo "$" #=> "hELLO WORLD!" (lowercase 1st letter) echo "$" #=> "hello world!" (all lowercase) str="hello world!" echo "$" #=> "Hello world!" (uppercase 1st letter) echo "$" #=> "HELLO WORLD!" (all uppercase)
Default values
| Expression | Description |
| $ | $foo , or val if unset (or null) |
| $ | Set $foo to val if unset (or null) |
| $ | val if $foo is set (and not null) |
| $ | Show error message and exit if $foo is unset (or null) |
Omitting the : removes the (non)nullity checks, e.g. $ expands to val if unset otherwise $foo .
Loops
Basic for loop
for i in /etc/rc.*; do echo "$i" done
C-like for loop
Ranges
for i in ; do echo "Welcome $i" done
With step size
for i in ; do echo "Welcome $i" done
Reading lines
while read -r line; do echo "$line" done
Forever
Functions
Defining functions
# Same as above (alternate syntax) function myfunc()
Returning values
Raising errors
if myfunc; then echo "success" else echo "failure" fi
Arguments
| Expression | Description |
| $# | Number of arguments |
| $* | All positional arguments (as a single word) |
| $@ | All positional arguments (as separate strings) |
| $1 | First argument |
| $_ | Last argument of the previous command |
Note: $@ and $* must be quoted in order to perform as described. Otherwise, they do exactly the same thing (arguments as separate strings).
Conditionals
Conditions
Note that [[ is actually a command/program that returns either 0 (true) or 1 (false). Any program that obeys the same logic (like all base utils, such as grep(1) or ping(1) ) can be used as condition, see examples.
| Condition | Description |
| [[ -z STRING ]] | Empty string |
| [[ -n STRING ]] | Not empty string |
| [[ STRING == STRING ]] | Equal |
| [[ STRING != STRING ]] | Not Equal |
| [[ NUM -eq NUM ]] | Equal |
| [[ NUM -ne NUM ]] | Not equal |
| [[ NUM -lt NUM ]] | Less than |
| [[ NUM -le NUM ]] | Less than or equal |
| [[ NUM -gt NUM ]] | Greater than |
| [[ NUM -ge NUM ]] | Greater than or equal |
| [[ STRING =~ STRING ]] | Regexp |
| (( NUM < NUM )) | Numeric conditions |
More conditions
| Condition | Description |
| [[ -o noclobber ]] | If OPTIONNAME is enabled |
| [[ ! EXPR ]] | Not |
| [[ X && Y ]] | And |
| [[ X || Y ]] | Or |
File conditions
| Condition | Description |
| [[ -e FILE ]] | Exists |
| [[ -r FILE ]] | Readable |
| [[ -h FILE ]] | Symlink |
| [[ -d FILE ]] | Directory |
| [[ -w FILE ]] | Writable |
| [[ -s FILE ]] | Size is > 0 bytes |
| [[ -f FILE ]] | File |
| [[ -x FILE ]] | Executable |
| [[ FILE1 -nt FILE2 ]] | 1 is more recent than 2 |
| [[ FILE1 -ot FILE2 ]] | 2 is more recent than 1 |
| [[ FILE1 -ef FILE2 ]] | Same files |
Example
# String if [[ -z "$string" ]]; then echo "String is empty" elif [[ -n "$string" ]]; then echo "String is not empty" else echo "This never happens" fi
# Combinations if [[ X && Y ]]; then . fi
if [[ -e "file.txt" ]]; then echo "file exists" fi
Arrays
Defining arrays
Fruits=('Apple' 'Banana' 'Orange')
Fruits[0]="Apple" Fruits[1]="Banana" Fruits[2]="Orange"
Working with arrays
echo "$" # Element #0 echo "$" # Last element echo "$" # All elements, space-separated echo "$" # Number of elements echo "$" # String length of the 1st element echo "$" # String length of the Nth element echo "$" # Range (from position 3, length 2) echo "$" # Keys of all elements, space-separated
Operations
Fruits=("$" "Watermelon") # Push Fruits+=('Watermelon') # Also Push Fruits=( "$" ) # Remove by regex match unset Fruits[2] # Remove one item Fruits=("$") # Duplicate Fruits=("$" "$") # Concatenate lines=(`cat "logfile"`) # Read from file
Iteration
Dictionaries
Defining
sounds[dog]="bark" sounds[cow]="moo" sounds[bird]="tweet" sounds[wolf]="howl"
Declares sound as a Dictionary object (aka associative array).
Working with dictionaries
echo "$" # Dog's sound echo "$" # All values echo "$" # All keys echo "$" # Number of elements unset sounds[dog] # Delete dog
Iteration
Iterate over values
for val in "$"; do echo "$val" done
Iterate over keys
for key in "$"; do echo "$key" done
Options
Options
set -o noclobber # Avoid overlay files (echo "hi" > foo) set -o errexit # Used to exit upon error, avoiding cascading errors set -o pipefail # Unveils hidden failures set -o nounset # Exposes unset variables
Glob options
shopt -s nullglob # Non-matching globs are removed ('*.foo' => '') shopt -s failglob # Non-matching globs throw errors shopt -s nocaseglob # Case insensitive globs shopt -s dotglob # Wildcards match dotfiles ("*.sh" => ".foo.sh") shopt -s globstar # Allow ** for recursive matches ('lib/**/*.rb' => 'lib/a/b/c.rb')
Set GLOBIGNORE as a colon-separated list of patterns to be removed from glob matches.
History
Commands
| Command | Description |
| history | Show history |
| shopt -s histverify | Don’t execute expanded result immediately |
Expansions
| Expression | Description |
| !$ | Expand last parameter of most recent command |
| !* | Expand all parameters of most recent command |
| !-n | Expand n th most recent command |
| !n | Expand n th command in history |
| ! | Expand most recent invocation of command |
Operations
| Code | Description |
| !! | Execute last command again |
| . s/// | Replace first occurrence of to in most recent command |
| . gs/// | Replace all occurrences of to in most recent command |
| !$:t | Expand only basename from last parameter of most recent command |
| !$:h | Expand only directory from last parameter of most recent command |
!! and !$ can be replaced with any valid expansion.
Slices
| Code | Description |
| . n | Expand only n th token from most recent command (command is 0 ; first argument is 1 ) |
| !^ | Expand first argument from most recent command |
| !$ | Expand last token from most recent command |
| . n-m | Expand range of tokens from most recent command |
| . n-$ | Expand n th token to last from most recent command |
!! can be replaced with any valid expansion i.e. !cat , !-2 , !42 , etc.
Miscellaneous
Numeric calculations
$(($RANDOM%200)) # Random number 0..199
declare -i count # Declare as type integer count+=1 # Increment
Subshells
(cd somedir; echo "I'm now in $PWD") pwd # still in first directory
Redirection
python hello.py > output.txt # stdout to (file) python hello.py >> output.txt # stdout to (file), append python hello.py 2> error.log # stderr to (file) python hello.py 2>&1 # stderr to stdout python hello.py 2>/dev/null # stderr to (null) python hello.py >output.txt 2>&1 # stdout and stderr to (file), equivalent to &> python hello.py &>/dev/null # stdout and stderr to (null) echo "$0: warning: too many users" >&2 # print diagnostic message to stderr
Inspecting commands
command -V cd #=> "cd is a function/alias/whatever"
Trap errors
trap 'echo Error at about $LINENO' ERR
traperr() < echo "ERROR: $at about $" > set -o errtrace trap traperr ERR
Case/switch
case "$1" in start | up) vagrant up ;; *) echo "Usage: $0 " ;; esac
Source relative
printf
printf "Hello %s, I'm %s" Sven Olga #=> "Hello Sven, I'm Olga printf "1 + 1 = %d" 2 #=> "1 + 1 = 2" printf "This is how you print a float: %f" 2 #=> "This is how you print a float: 2.000000" printf '%s\n' '#!/bin/bash' 'echo hello' >file # format string is applied to each group of arguments printf '%i+%i=%i\n' 1 2 3 4 5 9
| Command option | Description |
| -c | Operations apply to characters not in the given set |
| -d | Delete characters |
| -s | Replaces repeated characters with single occurrence |
| -t | Truncates |
| [:upper:] | All upper case letters |
| [:lower:] | All lower case letters |
| [:digit:] | All digits |
| [:space:] | All whitespace |
| [:alpha:] | All letters |
| [:alnum:] | All letters and digits |
Example
echo "Welcome To Devhints" | tr '[:lower:]' '[:upper:]' WELCOME TO DEVHINTS
Directory of script
Getting options
while [[ "$1" =~ ^- && ! "$1" == "--" ]]; do case $1 in -V | --version ) echo "$version" exit ;; -s | --string ) shift; string=$1 ;; -f | --flag ) flag=1 ;; esac; shift; done if [[ "$1" == '--' ]]; then shift; fi
Heredoc
echo -n "Proceed? [y/n]: " read -r ans echo "$ans"
The -r option disables a peculiar legacy behavior with backslashes.
read -n 1 ans # Just one character
Special variables
| Expression | Description |
| $? | Exit status of last task |
| $! | PID of last background task |
| $$ | PID of shell |
| $0 | Filename of the shell script |
| $_ | Last argument of the previous command |
| $ | return value of piped commands (array) |
Go to previous directory
pwd # /home/user/foo cd bar/ pwd # /home/user/foo/bar cd - pwd # /home/user/foo
Check for command’s result
if ping -c 1 google.com; then echo "It appears you have a working internet connection" fi
Grep check
if grep -q 'foo' ~/.bash_history; then echo "You appear to have typed 'foo' in the past" fi
Also see
- Bash-hackers wiki(bash-hackers.org)
- Shell vars(bash-hackers.org)
- Learn bash in y minutes(learnxinyminutes.com)
- Bash Guide(mywiki.wooledge.org)
- ShellCheck(shellcheck.net)
Источник