Bash Scripting
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Bash Scripting
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Example

#!/bin/bash
​
NAME="Payam"
echo "Hello $NAME!"
​
exit 0

Variables:

varname=value # defines a variable
varname=value command # defines a variable to be in the environment of a particular subprocess
echo $varname # checks a variable's value
read <varname> # reads a string from the input and assigns it to a variable
let <varname> = <equation> # performs mathematical calculation using operators like +, -, *, /, %
export VARNAME=value # defines an environment variable (will be available in subprocesses)
#Special shell variables
echo $ # prints process ID of the current shell
echo $! # prints process ID of the most recently invoked background job
echo $? # displays the exit status of the last command
echo $0 # display Filename of the shell script

Quoting:

\c #Take character c literally.
`cmd` #Run cmd and replace it in the line of code with its output.
"whatever" #Take whatever literally, after first interpreting $, `...`, \
'whatever' #Take whatever absolutely literally.
​
#Example:
match=`ls *.bak` #Puts names of .bak files into shell variable match.
echo \* #Echos * to screen, not all filename as in: echo *
echo '$1$2hello' #Writes literally $1$2hello on screen.
echo "$1$2hello" #Writes value of parameters 1 and 2 and string hello.

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

Brace expansion

{A,B} Same as A B
{A,B}.js Same as A.js B.js
{1..5} Same as 1 2 3 4 5

Parameter expansions

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"
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

${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

Length

${#FOO} Length of $FOO

Default Values

${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.

Comment

# Single line comment
: '
This is a
multi line
comment
'

Substrings

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

Manipulations

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)

Conditionals:

Test Operators

In Bash, the test command takes one of the following syntax forms:
  • test EXPRESSION
  • [ EXPRESSION ]
  • [[ EXPRESSION ]]
To make the script portable, prefer using the old test [ command which is available on all POSIX shells. The new upgraded version of the test command [[ (double brackets) is supported on most modern systems using Bash, Zsh, and Ksh as a default shell. To negate the test expression, use the logical NOT (!) operator.

Checking Numbers

Note that a shell variable could contain a string that represents a number. If you want to check the numerical value use one of the following:
[[ 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

Checking Strings

[[ -z STRING ]] Empty string
[[ -n STRING ]] Not empty string
[[ STRING == STRING ]] Equal
[[ STRING != STRING ]] Not Equal
​

Checking files

[[ -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

More conditions:

[[ -o noclobber ]] If OPTIONNAME is enabled
[[ ! EXPR ]] Not
[[ X && Y ]] And
[[ X || Y ]] Or

if statement:

#if Statement
​
echo -n "Enter a number: "
read VAR
​
if [[ $VAR -gt 10 ]]
then
echo "The variable is greater than 10."
fi
#if..else Statement
​
echo -n "Enter a number: "
read VAR
​
if [[ $VAR -gt 10 ]]
then
echo "The variable is greater than 10."
else
echo "The variable is equal or less than 10."
fi
#if..elif..else Statement
​
echo -n "Enter a number: "
read VAR
​
if [[ $VAR -gt 10 ]]
then
echo "The variable is greater than 10."
elif [[ $VAR -eq 10 ]]
then
echo "The variable is equal to 10."
else
echo "The variable is less than 10."
fi
# Nested if Statements
echo -n "Enter the first number: "
read VAR1
echo -n "Enter the second number: "
read VAR2
echo -n "Enter the third number: "
read VAR3
​
if [[ $VAR1 -ge $VAR2 ]]
then
if [[ $VAR1 -ge $VAR3 ]]
then
echo "$VAR1 is the largest number."
else
echo "$VAR3 is the largest number."
fi
else
if [[ $VAR2 -ge $VAR3 ]]
then
echo "$VAR2 is the largest number."
else
echo "$VAR3 is the largest number."
fi
fi

Loops:

for:

#basic for loop
for i in 1 2 3 4 5
do
echo "Welcome $i times"
done
#Basic for loop
for i in /etc/rc.*; do
echo $i
done
#Ranges
for i in {1..5}; do
echo "Welcome $i"
done
#C-Like for loop
for ((i = 0 ; i < 100 ; i++)); do
echo $i
done
#with step size
for i in {5..50..5}; do
echo "Welcome $i"
done

while:

n=1
​
while [ $n -le 5 ]
do
echo "Welcome $n times."
n=$(( n+1 ))
done
#Using ((expression)) Format With The While Loop
n=1
while (( $n <= 5 ))
do
echo "Welcome $n times."
n=$(( n+1 ))
done
#for ever
while true; do
···
done
# Reading a test file:
###example1/2:
cat /etc/resolv.conf | while read line; do
echo $line
done
​
###example2/2:
file=/etc/resolv.conf
while IFS= read -r line
do
echo $line
done < "$file"
​
### Reading A Text File With Separate Fields:
file=/etc/resolv.conf
# set field separator to a single white space
while IFS=' ' read -r f1 f2
do
echo "field # 1 : $f1 ==> field #2 : $f2"
done < "$file"

Until:

#!/bin/bash
​
counter=0
​
until [ $counter -gt 5 ]
do
echo Counter: $counter
((counter++))
done

Case:

Case/switch
case "$1" in
start | up)
vagrant up
;;
​
*)
echo "Usage: $0 {start|stop|ssh}"
;;
esac

Functions:

# 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:
$# Number of arguments
$* All arguments
[email protected] All arguments, starting from first
$1 First argument
$_ Last argument of the previous command

Arrays

Defining arrays
Fruits=('Apple' 'Banana' 'Orange')
Fruits[0]="Apple"
Fruits[1]="Banana"
Fruits[2]="Orange"
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
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 ${!Fruits[@]} # Keys of all elements, space-separated
Iteration
for i in "${arrayName[@]}"; do
echo $i
done

Dictionaries:

Defining
declare -A sounds
sounds[dog]="bark"
sounds[cow]="moo"
sounds[bird]="tweet"
sounds[wolf]="howl"
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

Debugging

bash -n scriptname # don't run commands; check for syntax errors only
set -o noexec # alternative (set option in script)
​
bash -v scriptname # echo commands before running them
set -o verbose # alternative (set option in script)
​
bash -x scriptname # echo commands after command-line processing
set -o xtrace # alternative (set option in script)

Miscellaneous:

#Numeric calculations
$((a + 200)) # Add 200 to $a
$(($RANDOM%200)) # Random number 0..199
#Inspecting commands
command -V cd
#=> "cd is a function/alias/whatever"
#Heredoc:
cat <<END
hello world
END
#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"
#Reading input
echo -n "Proceed? [y/n]: "
read ans
echo $ans
​
#Reading Just one character:
read -n 1 ans
#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
#Check for command’s result
if ping -c 1 google.com; then
echo "It appears you have a working internet connection"
fi
​
Payam Borosan.Goodluck
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Refrences
Last modified 2yr ago
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On this page
Example
Variables:
Quoting:
Redirection
Brace expansion
Parameter expansions
Basics
Substitution
Length
Default Values
Comment
Substrings
Manipulations
Conditionals:
Test Operators
Checking Numbers
Checking Strings
Checking files
More conditions:
if statement:
Loops:
for:
while:
Until:
Case:
Functions:
Arrays
Dictionaries:
Debugging
Miscellaneous: