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Bash scripting
CLI
2017/sheet
Featured
2017-08-26
One-page reference on Bash shell scripting. Learn about variables, functions, errors, process ID's with examples and more.

Getting started

{: .-three-column}

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

See Command substitution

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 "$string" ]; then
  echo "String is empty"
elsif [ -n "$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

| {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

{: .-three-column}

Basics

name="John"
echo ${name}
echo ${name/J/j}    #=> "john" (substitution)
echo ${name:0:2}    #=> "jo" (slicing)
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##*.}     # 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 ${STR:-5:5}  # "world"
SRC="/path/to/foo.cpp"
BASE=${STR##*/}   #=> "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

Substrings

| ${FOO:0:3} | Substring (position, length) | | ${FOO:-3:3} | Substring from the right |

Length

| ${#FOO} | Length of $FOO |

Default values

| ${FOO:-val} | $FOO, or val if not set | | ${FOO:=val} | Set $FOO to val if not set | | ${FOO:+val} | val if $FOO is set | | ${FOO:?message} | Show error message and exit if $FOO is not set |

The : is optional (eg, ${FOO=word} works)

Loops

{: .-three-column}

Basic for loop

for i in /etc/rc.*; do
  echo $i
done

Ranges

for i in {1..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 arguments
$@ All arguments, starting from first
$1 First argument

See Special parameters.

Conditionals

{: .-three-column}

Conditions

Condition Description
[ -z STRING ] Empty string
[ -n STRING ] Not empty string
--- ---
[ 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
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 "$string" ]; then
  echo "String is empty"
elsif [ -n "$string" ]; then
  echo "String is not empty"
fi
# Combinations
if [ X ] && [ Y ]; then
  ...
fi
# Regex
if [[ "A" =~ "." ]]
if (( $a < $b ))
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[@]}           # 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)

Operations

Fruits=("${Fruits[@]}" "Watermelon")    # 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

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

set -o nullglob    # Non-matching globs are removed  ('*.foo' => '')
set -o failglob    # Non-matching globs throw errors
set -o nocaseglob  # Case insensitive globs
set -o globdots    # Wildcards match dotfiles ("*.sh" => ".foo.sh")
set -o 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.

Miscellaneous

Numeric calculations

$((a + 200))      # Add 200 to $a
$((RANDOM%=200))  # Random number 0..200

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 < foo.txt

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

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

Process IDs

| $? | PID of last foreground task | | $! | PID of last background task | | $$ | PID of shell |

See Special parameters.

Also see

{: .-one-column}