Getting started
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Introduction
{: .-intro}
This is a quick reference to getting started with Bash scripting.
- Learn bash in y minutes (learnxinyminutes.com)
- Bash Guide (mywiki.wooledge.org)
Example
#!/usr/bin/env bash
NAME="John"
echo "Hello $NAME!"
Variables
NAME="John"
echo $NAME
echo "$NAME"
echo "${NAME}!"
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`"
# Same
Conditional execution
git commit && git push
git commit || echo "Commit failed"
Functions
{: id=‘functions-example’}
get_name() {
echo "John"
}
echo "You are $(get_name)"
See: Functions
Conditionals
{: id=‘conditionals-example’}
if [[ -z "$string" ]]; then
echo "String is empty"
elif [[ -n "$string" ]]; then
echo "String is not empty"
fi
See: Conditionals
Strict mode
set -euo pipefail
IFS=$'\n\t'
See: Unofficial bash strict mode
Brace expansion
echo {A,B}.js
| Expression | Description |
|---|---|
{A,B} | Same as A B |
{A,B}.js | Same as A.js B.js |
{1..5} | Same as 1 2 3 4 5 |
See: Brace expansion
Parameter expansions
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Basics
name="John"
echo ${name}
echo ${name/J/j} #=> "john" (substitution)
echo ${name:0:2} #=> "Jo" (slicing)
echo ${name::2} #=> "Jo" (slicing)
echo ${name::-1} #=> "Joh" (slicing)
echo ${name:(-1)} #=> "n" (slicing from right)
echo ${name:(-2):1} #=> "h" (slicing from right)
echo ${food:-Cake} #=> $food or "Cake"
length=2
echo ${name:0:length} #=> "Jo"
See: Parameter expansion
STR="/path/to/foo.cpp"
echo ${STR%.cpp} # /path/to/foo
echo ${STR%.cpp}.o # /path/to/foo.o
echo ${STR%/*} # /path/to
echo ${STR##*.} # cpp (extension)
echo ${STR##*/} # foo.cpp (basepath)
echo ${STR#*/} # path/to/foo.cpp
echo ${STR##*/} # foo.cpp
echo ${STR/foo/bar} # /path/to/bar.cpp
STR="Hello world"
echo ${STR:6:5} # "world"
echo ${STR: -5:5} # "world"
SRC="/path/to/foo.cpp"
BASE=${SRC##*/} #=> "foo.cpp" (basepath)
DIR=${SRC%$BASE} #=> "/path/to/" (dirpath)
Substitution
| Code | Description |
|---|---|
${FOO%suffix} | Remove suffix |
${FOO#prefix} | Remove prefix |
| — | — |
${FOO%%suffix} | Remove long suffix |
${FOO##prefix} | Remove long prefix |
| — | — |
${FOO/from/to} | Replace first match |
${FOO//from/to} | Replace all |
| — | — |
${FOO/%from/to} | Replace suffix |
${FOO/#from/to} | Replace prefix |
Comments
# Single line comment
: '
This is a
multi line
comment
'
Substrings
| Expression | Description |
|---|---|
${FOO:0:3} | Substring (position, length) |
${FOO:(-3):3} | Substring from the right |
Length
| Expression | Description |
|---|---|
${#FOO} | Length of $FOO |
Manipulation
STR="HELLO WORLD!"
echo ${STR,} #=> "hELLO WORLD!" (lowercase 1st letter)
echo ${STR,,} #=> "hello world!" (all lowercase)
STR="hello world!"
echo ${STR^} #=> "Hello world!" (uppercase 1st letter)
echo ${STR^^} #=> "HELLO WORLD!" (all uppercase)
Default values
| Expression | Description |
|---|---|
${FOO:-val} | $FOO, or val if unset (or null) |
${FOO:=val} | Set $FOO to val if unset (or null) |
${FOO:+val} | val if $FOO is set (and not null) |
${FOO:?message} | Show error message and exit if $FOO is unset (or null) |
Omitting the : removes the (non)nullity checks, e.g. ${FOO-val} expands to val if unset otherwise $FOO.
Loops
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Basic for loop
for i in /etc/rc.*; do
echo $i
done
C-like for loop
for ((i = 0 ; i < 100 ; i++)); do
echo $i
done
Ranges
for i in {1..5}; do
echo "Welcome $i"
done
With step size
for i in {5..50..5}; do
echo "Welcome $i"
done
Reading lines
cat file.txt | while read line; do
echo $line
done
Forever
while true; do
···
done
Functions
{: .-three-column}
Defining functions
myfunc() {
echo "hello $1"
}
# Same as above (alternate syntax)
function myfunc() {
echo "hello $1"
}
myfunc "John"
Returning values
myfunc() {
local myresult='some value'
echo $myresult
}
result="$(myfunc)"
Raising errors
myfunc() {
return 1
}
if myfunc; then
echo "success"
else
echo "failure"
fi
Arguments
| Expression | Description |
|---|---|
$# | Number of arguments |
$* | All postional arguments (as a single word) |
$@ | All postitional arguments (as separate strings) |
$1 | First argument |
$_ | Last argument of the previous command |
Note: $@ and $* must be quoted in order to perform as described.
Otherwise, they do exactly the same thing (arguments as separate strings).
See Special parameters.
Conditionals
{: .-three-column}
Conditions
Note that [[ is actually a command/program that returns either 0 (true) or 1 (false). Any program that obeys the same logic (like all base utils, such as grep(1) or ping(1)) can be used as condition, see examples.
| Condition | Description |
|---|---|
[[ -z STRING ]] | Empty string |
[[ -n STRING ]] | Not empty string |
[[ STRING == STRING ]] | Equal |
[[ STRING != STRING ]] | Not Equal |
| — | — |
[[ NUM -eq NUM ]] | Equal |
[[ NUM -ne NUM ]] | Not equal |
[[ NUM -lt NUM ]] | Less than |
[[ NUM -le NUM ]] | Less than or equal |
[[ NUM -gt NUM ]] | Greater than |
[[ NUM -ge NUM ]] | Greater than or equal |
| — | — |
[[ STRING =~ STRING ]] | Regexp |
| — | — |
(( NUM < NUM )) | Numeric conditions |
More conditions
| Condition | Description |
|---|---|
[[ -o noclobber ]] | If OPTIONNAME is enabled |
| — | — |
[[ ! EXPR ]] | Not |
[[ X && Y ]] | And |
| `[[ X |
File conditions
| Condition | Description |
|---|---|
[[ -e FILE ]] | Exists |
[[ -r FILE ]] | Readable |
[[ -h FILE ]] | Symlink |
[[ -d FILE ]] | Directory |
[[ -w FILE ]] | Writable |
[[ -s FILE ]] | Size is > 0 bytes |
[[ -f FILE ]] | File |
[[ -x FILE ]] | Executable |
| — | — |
[[ FILE1 -nt FILE2 ]] | 1 is more recent than 2 |
[[ FILE1 -ot FILE2 ]] | 2 is more recent than 1 |
[[ FILE1 -ef FILE2 ]] | Same files |
Example
# String
if [[ -z "$string" ]]; then
echo "String is empty"
elif [[ -n "$string" ]]; then
echo "String is not empty"
else
echo "This never happens"
fi
# Combinations
if [[ X && Y ]]; then
...
fi
# Equal
if [[ "$A" == "$B" ]]
# Regex
if [[ "A" =~ . ]]
if (( $a < $b )); then
echo "$a is smaller than $b"
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 ${Fruits[0]} # Element #0
echo ${Fruits[-1]} # Last element
echo ${Fruits[@]} # All elements, space-separated
echo ${#Fruits[@]} # Number of elements
echo ${#Fruits} # String length of the 1st element
echo ${#Fruits[3]} # String length of the Nth element
echo ${Fruits[@]:3:2} # Range (from position 3, length 2)
echo ${!Fruits[@]} # Keys of all elements, space-separated
Operations
Fruits=("${Fruits[@]}" "Watermelon") # Push
Fruits+=('Watermelon') # Also Push
Fruits=( ${Fruits[@]/Ap*/} ) # Remove by regex match
unset Fruits[2] # Remove one item
Fruits=("${Fruits[@]}") # Duplicate
Fruits=("${Fruits[@]}" "${Veggies[@]}") # Concatenate
lines=(`cat "logfile"`) # Read from file
Iteration
for i in "${arrayName[@]}"; do
echo $i
done
Dictionaries
{: .-three-column}
Defining
declare -A sounds
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 ${sounds[dog]} # Dog's sound
echo ${sounds[@]} # All values
echo ${!sounds[@]} # All keys
echo ${#sounds[@]} # Number of elements
unset sounds[dog] # Delete dog
Iteration
Iterate over values
for val in "${sounds[@]}"; do
echo $val
done
Iterate over keys
for key in "${!sounds[@]}"; 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 nth most recent command |
!n | Expand nth command in history |
!<command> | Expand most recent invocation of command <command> |
Operations
| Code | Description |
|---|---|
!! | Execute last command again |
!!:s/<FROM>/<TO>/ | Replace first occurrence of <FROM> to <TO> in most recent command |
!!:gs/<FROM>/<TO>/ | Replace all occurrences of <FROM> to <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 nth 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 nth token to last from most recent command |
!! can be replaced with any valid expansion i.e. !cat, !-2, !42, etc.
Miscellaneous
Numeric calculations
$((a + 200)) # Add 200 to $a
$(($RANDOM%200)) # Random number 0..199
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 &>/dev/null # stdout and stderr to (null)
python hello.py < foo.txt # feed foo.txt to stdin for python
Inspecting commands
command -V cd
#=> "cd is a function/alias/whatever"
Trap errors
trap 'echo Error at about $LINENO' ERR
or
traperr() {
echo "ERROR: ${BASH_SOURCE[1]} at about ${BASH_LINENO[0]}"
}
set -o errtrace
trap traperr ERR
Case/switch
case "$1" in
start | up)
vagrant up
;;
*)
echo "Usage: $0 {start|stop|ssh}"
;;
esac
Source relative
source "${0%/*}/../share/foo.sh"
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"
Directory of script
DIR="${0%/*}"
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
cat <<END
hello world
END
Reading input
echo -n "Proceed? [y/n]: "
read ans
echo $ans
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 |
See Special parameters.
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)
