Bash Tips

Printf

printf "%s\n" print arguments on "separate lines"

print
arguments
on
separate
lines

printf "%b\n" "Hello\nWorld" "12\twords"

Hello
World
12      words

printf "%d\n" 23 45 56.78 0xff 011

23
45
-bash: printf: 56.78: invalid number
56
255
9

Note

Using a leading 0 is going to be octal. Using a lead 0x is going to be hexadecimal.

printf "%f\n" 12.24 23 56.789 1.2345678

printf "%e\n" 12.34 23

1.234000e+01
2.300000e+01

Width

Note

- here in output means space.

printf "%8s %10s\n" nima saed

----nima ------saed

printf "%8s %-10s\n" nima saed

----nima saed------

printf "%8.2s %-10s\n" nima saed

------ni saed------

printf "%4d\n" 23 54

--23
--54

printf "%04d\n" 23 54

0023
0054

printf "%10.4d\n" 23 54

------0023
------0054

printf "%10.4f\n" 23 54

---23.0000
---54.0000

read command

read a b "test test1 test2"

echo $a
echo $b

test
test1 test2

while loop

while
    command lists
do
    command lists
done

examples

while
    ((x<10))
do
    echo loop $x;
    date > date.$x;
    ((x=x+1))
done

while
    read a b
do
    echo a is $a b is $b
done <data_file

ls -l | while read a b c d; do echo owner is $c; done

For Loops

for <var> in <list>
do
    comand lists
done

Examples

for i in dog cat elephant
do
    echo i is $i
done

for i in dog cat elephant; do echo i is $i; done

seq

seq 1 5

for num in `seq 1 5`; do echo $num; done

Note

You can also use {A..Z}, {a..f} or {1..10} in for loops.

for i in {a..f}; do echo $i; done

a
b
c
d
e
f

Dealing with files

for d in $(<data_file); do echo $d; done

for j in *.c
# making a list with file globbing
do
    command
done

for f in $(find . -name *.c)
do
 command
done

List/Array

for n in ${list[@]}; do echo $n; done

Functions

function name{
    command list
}

function myfunc {
    echo starting
    return
    echo this will not be executed
}

Export functions

export -f myfunc

Case Statement

case expression in
pattern 1 )
    command list ;;
pattern 2 )
    command list ;;
esac

case $ans in
yes|YES|y|Y|y.x) echo "Will do!";;
n*|N*) echo "Will NOT do!";;
*) echo "Oops!";;
esac

If-Then-Else Statement

if
    command list # last result is used
    then
    command list
    [else
    command list]
fi

Example

if
grep -q important myfile
then
    echo myfile has important stuff
else
    echo myfile does not have important stuff
fi

Extra

File name and path

Note

This code is inside the script file.

To get name of script file

echo $0

To get full path of the file

echo $(readlink -f $0)

To get full path but move one directory up

echo $(dirname "$(readlink -f $0)")

Progress bar

echo -ne '#####                     (33%)\r'
sleep 1
echo -ne '#############             (66%)\r'
sleep 1
echo -ne '#######################   (100%)\r'
echo -ne '\n'

Bash parsing and expansion

Brace expansion

{one,two,three}
{1..10} or {a..z}
pre{d,l}ate
{{1..3},{a..c}} => 1,2,3,a,b,c
{1..3}{a..c} => 1a,1b,1c,2a,...,3c
{01..13..3} => 01,04,07,10,13
{a..h..3} => a,d,g

Tilde expansion

~ => /home/nima
~someone => /home/someone
~noUser => ~noUser

Parameter and variable expansion

var=whatever
$var or ${var} => whatever
echo $var “$var” #are having different result  if there are white spaces

Arithmetic expansion

$(( expression ))

Command substitution

wc -l $( date +%Y-%m-%d).log

Note

If command substitution is not quoted, word splitting and pathname expansion are performed on the result.

Word splitting

The result of parameter and arithmetic expansion, as well as command substitution, are subject to word splitting if the were NOT QUETED

var="this is a multi-word value"
printf ":%s:\n" $var

:this:
:is:
:a:
:multi-word:
:value:

var="this is a multi-word value"
printf ":%s:\n" $var

:this is a multi-word value:

Note

Word splitting is based on value of IFS (Internal Field Separator). IFS default value is space, tab, and newline. (IFS=$' \t\n'). You can change IFS value to any value.

Pathname expansion

Char in use are *, ?, and []

echo D*

list files starting with capital D.

echo ?a*

list files which the second char is a.

echo *[0-9]*

list files which there is at least one number in there name.

Process substitution

<(command)
>(command)

Parameter Expansion

empty or unset variable

Default
- echo {var:-default} => default if it is not set and empty string
- echo {var-default} => default if it is not set
Alternative
- echo {var:+alter} => alter if it is set and not empty string
- echo {var+alter} => alter if it is set
Default with assignment
- echo {var:=default} => default if it is not set and empty string, assign default to var as well
- echo {var=default} => default if it is not set, assign default to var as well.
Message
- echo {var:?message} => display error message if it is not set and empty string
- echo {var?message} => display error message if it is not set

** Length of var

var=test
echo ${#var}

Remove Pattern

Short from end

var=Toronto
echo ${var%o*}

Toront

Long from end

var=Toronto
echo ${var%%o*}

Short from beginning

var=Toronto
echo ${var#*o}

ronto

Long from beginning

scriptname=${0##*/} ## /home/user/script.sh => script.sh

Replace pattern

password=reretrgfdsgdhdt
echo "${password//?/*}"

*************

Substring var:offset:length

var=Toronto
echo "${var:3:2}"

on

var=Toronto
echo "${var:1}"

oronto

Negative offset

var=Toronto
echo "${var: -3}" # Do not forget about space betwen : and -

nto