title: Bash date: 2020-11-25 18:28:43 background: bg-[#3e4548] tags: - shell - sh - echo - script - linux categories: - Programming
#!/bin/bash
VAR="world"
echo "Hello $VAR!" # => Hello world!
Execute the script ```shell script $ bash hello.sh
### Variables
```bash
NAME="John"
echo ${NAME} # => John (Variables)
echo $NAME # => John (Variables)
echo "$NAME" # => John (Variables)
echo '$NAME' # => $NAME (Exact string)
echo "${NAME}!" # => John! (Variables)
NAME = "John" # => Error (about space)
# This is an inline Bash comment.
: '
This is a
very neat comment
in bash
'
Multi-line comments use :'
to open and '
to close
Expression | Description |
---|---|
$1 … $9 |
Parameter 1 ... 9 |
$0 |
Name of the script itself |
$1 |
First argument |
${10} |
Positional parameter 10 |
$# |
Number of arguments |
$$ |
Process id of the shell |
$* |
All arguments |
$@ |
All arguments, starting from first |
$- |
Current options |
$_ |
Last argument of the previous command |
See: Special parameters
get_name() {
echo "John"
}
echo "You are $(get_name)"
See: Functions
if [[ -z "$string" ]]; then
echo "String is empty"
elif [[ -n "$string" ]]; then
echo "String is not empty"
fi
See: Conditionals
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
# => I'm in /path/of/current
echo "I'm in $(PWD)"
# Same as:
echo "I'm in `pwd`"
See: Command 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 |
Expression | Description |
---|---|
${FOO:0:3} |
Substring (position, length) |
${FOO:(-3):3} |
Substring from the right |
Expression | Description |
---|---|
${#FOO} |
Length of $FOO |
Expression | Description |
---|---|
${FOO:-val} |
$FOO , or val if unset |
${FOO:=val} |
Set $FOO to val if unset |
${FOO:+val} |
val if $FOO is set |
${FOO:?message} |
Show message and exit if $FOO is unset |
echo ${food:-Cake} #=> $food or "Cake"
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
name="John"
echo ${name} # => John
echo ${name:0:2} # => Jo
echo ${name::2} # => Jo
echo ${name::-1} # => Joh
echo ${name:(-1)} # => n
echo ${name:(-2)} # => hn
echo ${name:(-2):2} # => hn
length=2
echo ${name:0:length} # => Jo
See: Parameter expansion
SRC="/path/to/foo.cpp"
BASEPATH=${SRC##*/}
echo $BASEPATH # => "foo.cpp"
DIRPATH=${SRC%$BASEPATH}
echo $DIRPATH # => "/path/to/"
STR="HELLO WORLD!"
echo ${STR,} # => hELLO WORLD!
echo ${STR,,} # => hello world!
STR="hello world!"
echo ${STR^} # => Hello world!
echo ${STR^^} # => HELLO WORLD!
ARR=(hello World)
echo "${ARR[@],}" # => hello world
echo "${ARR[@]^}" # => Hello World
Fruits=('Apple' 'Banana' 'Orange')
Fruits[0]="Apple"
Fruits[1]="Banana"
Fruits[2]="Orange"
ARRAY1=(foo{1..2}) # => foo1 foo2
ARRAY2=({A..D}) # => A B C D
# Merge => foo1 foo2 A B C D
ARRAY3=(${ARRAY1[@]} ${ARRAY2[@]})
# declare construct
declare -a Numbers=(1 2 3)
Numbers+=(4 5) # Append => 1 2 3 4 5
-------------------- | --------------- |
${Fruits[0]} |
First element |
${Fruits[-1]} |
Last element |
${Fruits[*]} |
All elements |
${Fruits[@]} |
All elements |
${#Fruits[@]} |
Number of all |
${#Fruits} |
Length of 1st |
${#Fruits[3]} |
Length of nth |
${Fruits[@]:3:2} |
Range |
${!Fruits[@]} |
Keys of all |
Fruits=('Apple' 'Banana' 'Orange')
for e in "${Fruits[@]}"; do
echo $e
done
for i in "${!Fruits[@]}"; do
printf "%s\t%s\n" "$i" "${Fruits[$i]}"
done
Fruits=("${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
function extract()
{
local -n myarray=$1
local idx=$2
echo "${myarray[$idx]}"
}
Fruits=('Apple' 'Banana' 'Orange')
extract Fruits 2 # => Orangle
declare -A sounds
sounds[dog]="bark"
sounds[cow]="moo"
sounds[bird]="tweet"
sounds[wolf]="howl"
echo ${sounds[dog]} # Dog's sound
echo ${sounds[@]} # All values
echo ${!sounds[@]} # All keys
echo ${#sounds[@]} # Number of elements
unset sounds[dog] # Delete dog
for val in "${sounds[@]}"; do
echo $val
done
for key in "${!sounds[@]}"; do
echo $key
done
Condition | Description |
---|---|
[[ NUM -eq NUM ]] |
|
[[ NUM -ne NUM ]] |
|
[[ NUM -lt NUM ]] |
|
[[ NUM -le NUM ]] |
|
[[ NUM -gt NUM ]] |
|
[[ NUM -ge NUM ]] |
|
(( NUM < NUM )) |
Less than |
(( NUM <= NUM )) |
Less than or equal |
(( NUM > NUM )) |
Greater than |
(( NUM >= NUM )) |
Greater than or equal |
Condition | Description |
---|---|
[[ -z STR ]] |
Empty string |
[[ -n STR ]] |
|
[[ STR == STR ]] |
Equal |
[[ STR = STR ]] |
Equal (Same above) |
[[ STR < STR ]] |
Less than (ASCII) |
[[ STR > STR ]] |
Greater than (ASCII) |
[[ STR != STR ]] |
Not Equal |
[[ STR =~ STR ]] |
Regexp |
if [[ -z "$string" ]]; then
echo "String is empty"
elif [[ -n "$string" ]]; then
echo "String is not empty"
else
echo "This never happens"
fi
if [[ X && Y ]]; then
...
fi
if [[ "$A" == "$B" ]]; then
...
fi
if [[ '1. abc' =~ ([a-z]+) ]]; then
echo ${BASH_REMATCH[1]}
fi
if (( $a < $b )); then
echo "$a is smaller than $b"
fi
if [[ -e "file.txt" ]]; then
echo "file exists"
fi
Condition | Description |
---|---|
[[ -e FILE ]] |
|
[[ -d FILE ]] |
|
[[ -f FILE ]] |
|
[[ -h FILE ]] |
Symlink |
[[ -s FILE ]] |
Size is > 0 bytes |
[[ -r FILE ]] |
|
[[ -w FILE ]] |
|
[[ -x FILE ]] |
Executable |
[[ f1 -nt f2 ]] |
f1 |
[[ f1 -ot f2 ]] |
f2 |
[[ f1 -ef f2 ]] |
Same files |
Condition | Description |
---|---|
[[ -o noclobber ]] |
If OPTION is enabled |
[[ ! EXPR ]] |
Not |
[[ X && Y ]] |
And |
[[ X || Y ]] |
Or |
if [ "$1" = 'y' -a $2 -gt 0 ]; then
echo "yes"
fi
if [ "$1" = 'n' -o $2 -lt 0 ]; then
echo "no"
fi
for i in /etc/rc.*; do
echo $i
done
for ((i = 0 ; i < 100 ; i++)); do
echo $i
done
for i in {1..5}; do
echo "Welcome $i"
done
for i in {5..50..5}; do
echo "Welcome $i"
done
i=1
while [[ $i -lt 4 ]]; do
echo "Number: $i"
((i++))
done
i=3
while [[ $i -gt 0 ]]; do
echo "Number: $i"
((i--))
done
```bash {data=3,5} for number in \((seq 1 3); do if [[ \)number == 2 ]]; then continue; fi echo "$number" done
### Break
```bash
for number in $(seq 1 3); do
if [[ $number == 2 ]]; then
# Skip entire rest of loop.
break;
fi
# This will only print 1
echo "$number"
done
count=0
until [ $count -gt 10 ]; do
echo "$count"
((count++))
done
while true; do
# here is some code.
done
while :; do
# here is some code.
done
cat file.txt | while read line; do
echo $line
done
myfunc() {
echo "hello $1"
}
# Same as above (alternate syntax)
function myfunc() {
echo "hello $1"
}
myfunc "John"
myfunc() {
local myresult='some value'
echo $myresult
}
result="$(myfunc)"
myfunc() {
return 1
}
if myfunc; then
echo "success"
else
echo "failure"
fi
# Avoid overlay files
# (echo "hi" > foo)
set -o noclobber
# Used to exit upon error
# avoiding cascading errors
set -o errexit
# Unveils hidden failures
set -o pipefail
# Exposes unset variables
set -o nounset
# Non-matching globs are removed
# ('*.foo' => '')
shopt -s nullglob
# Non-matching globs throw errors
shopt -s failglob
# Case insensitive globs
shopt -s nocaseglob
# Wildcards match dotfiles
# ("*.sh" => ".foo.sh")
shopt -s dotglob
# Allow ** for recursive matches
# ('lib/**/*.rb' => 'lib/a/b/c.rb')
shopt -s globstar
Command | Description |
---|---|
history |
Show history |
sudo !! |
Run the previous command with sudo |
shopt -s histverify |
Don't execute expanded result immediately |
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 |
!<command> |
Expand most recent invocation of command <command> |
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.
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.
$((a + 200)) # Add 200 to $a
$(($RANDOM%200)) # Random number 0..199
(cd somedir; echo "I'm now in $PWD")
pwd # still in first directory
command -V cd
#=> "cd is a function/alias/whatever"
python hello.py > output.txt # stdout to (file)
python hello.py >> output.txt # stdout to (file), append
python hello.py 2> error.log # stderr to (file)
python hello.py 2>&1 # stderr to stdout
python hello.py 2>/dev/null # stderr to (null)
python hello.py &>/dev/null # stdout and stderr to (null)
python hello.py < foo.txt # feed foo.txt to stdin for python
source "${0%/*}/../share/foo.sh"
DIR="${0%/*}"
case "$1" in
start | up)
vagrant up
;;
*)
echo "Usage: $0 {start|stop|ssh}"
;;
esac
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
printf "Hello %s, I'm %s" Sven Olga
#=> "Hello Sven, I'm Olga
printf "1 + 1 = %d" 2
#=> "1 + 1 = 2"
printf "Print a float: %f" 2
#=> "Print a float: 2.000000"
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
if ping -c 1 google.com; then
echo "It appears you have a working internet connection"
fi
Expression | Description |
---|---|
$? |
Exit status of last task |
$! |
PID of last background task |
$$ |
PID of shell |
$0 |
Filename of the shell script |
See Special parameters.
if grep -q 'foo' ~/.bash_history; then
echo "You appear to have typed 'foo' in the past"
fi
Escape these special characters with \
cat <<END
hello world
END
pwd # /home/user/foo
cd bar/
pwd # /home/user/foo/bar
cd -
pwd # /home/user/foo
echo -n "Proceed? [y/n]: "
read ans
echo $ans
read -n 1 ans # Just one character
git commit && git push
git commit || echo "Commit failed"
set -euo pipefail
IFS=$'\n\t'
See: Unofficial bash strict mode
args=("$@")
args+=(foo)
args+=(bar)
echo "${args[@]}"
Put the arguments into an array and then append