How To Use Bash Parameter Substitution Like A Pro

Posted on in Categories Linux, Shell scripting, UNIX last updated June 14, 2017

The $ character is used for parameter expansion, arithmetic expansion and command substitution. You can use it for manipulating and expanding variables on demands without using external commands such as perl, python, sed or awk. This guide shows you how to use parameter expansion modifiers to transform Bash shell variables for your scripting needs.

Parameter expansion in Bash

Syntax

You can use variables to store data and configuration options. For example:
dest="/backups"
Use echo or printf command to display variable value:
echo "$dest"
OR
printf "$dest\n"
The parameter name or symbol such as $dest to be expanded may be enclosed in braces
echo "Value ${dest}"
It is optional but serve to protect the variable to be expanded from characters immediately following it which could be interpreted as part of the name.

#1: Setting Up Default Shell Variables Value

The syntax is as follows:

${parameter:-defaultValue}
var=${parameter:-defaultValue}

If parameter not set, use defaultValue. In this example, your shell script takes arguments supplied on the command line. You’d like to provide default value so that the most common value can be used without needing to type them every time. If variable $1 is not set or passed, use root as default value for u:

u=${1:-root}

Consider the following example:

#!/bin/bash
_jail_dir="${1:-/home/phpcgi}"
echo "Setting php-cgi at ${_jail_dir}..."
# rest of the script ...

You can now run this script as follows:

./script.sh /jail              # <--- set php jail at /jail dir
./script.sh /home/httpd/jail   # <---- set php jail at /home/httpd/jail dir
./script.sh                    # <--- set php jail dir at /home/phpcgi (default)

Here is another handy example:

_mkdir(){
        local d="$1"               # get dir name
        local p=${2:-0755}      # get permission, set default to 0755
        [ $# -eq 0 ] && { echo "$0: dirname"; return; }
        [ ! -d "$d" ] && mkdir -m $p -p "$d"
}

Use this substitution for creating failsafe functions and providing missing command line arguments in scripts.

#1.1: Setting Default Values

The syntax is as follows:

${var:=value}
var=${USER:=value}

The assignment (:=) operator is used to assign a value to the variable if it doesn’t already have one. Try the following examples:

echo $USER

Sample outputs:

vivek

Now, assign a value foo to the $USER variable if doesn’t already have one:

echo ${USER:=foo}

Sample outputs:

vivek

Unset value for $USER:

unset USER
echo ${USER:=foo}

Sample outputs:

foo

This make sure you always have a default reasonable value for your script.

Tip: ${var:-defaultValue} vs ${var:=defaultValue}

Please note that it will not work with positional parameter arguments:

var=${1:=defaultValue}  ### FAIL with an error cannot assign in this way
var=${1:-defaultValue}    ### Perfect

#2: Display an Error Message If $VAR Not Passed

If the variable is not defined or not passed, you can stop executing the Bash script with the following syntax:

${varName?Error varName is not defined}
${varName:?Error varName is not defined or is empty}
${1:?"mkjail: Missing operand"}
MESSAGE="Usage: mkjail.sh domainname IPv4"             ### define error message
_domain=${2?"Error: ${MESSAGE}"}  ### you can use $MESSAGE too

This is used for giving an error message for unset parameters. In this example, if the $1 command line arg is not passed, stop executing the script with an error message:

_domain="${1:?Usage: mknginxconf domainName}"

Here is a sample script:

#!/bin/bash
# Purpose: Wrapper script to setup Nginx Load Balancer
# Author: Vivek Gite
_root="/nas.pro/prod/scripts/perl/nginx"
_setup="${_root}/createnginxconf.pl"
_db="${_root}/database/text/vips.db"
_domain="${1:?Usage: mknginxconf domainName}"     ### die if domainName is not passed ####
 
[ ! -f $_db ] && { echo "$0: Error $_db file not found."; exit 1; }
line=$(grep "^${_domain}" $_db) || { echo "$0: Error $_domain not found in $_db."; exit 2; }
 
# Get domain config info into 4 fields:
# f1 - Domain Name|
# f2 - IPv4Vip:httpPort:HttpsPort, IPv6Vip:httpPort:HttpsPort|
# f3 - PrivateIP1:port1,PrivateIP2,port2,...PrivateIPN,portN|
# f4 - LB Type (true [round robin] OR false [session])
# -------------------------------------------------------------------------------
IFS='|'
read -r f1 f2 f3 f4  <<<"$line"
 
# Do we want ssl host config too?
IFS=':'
set -- $f2
ssl="false"
[ "$3" == "443" ] && ssl="true"
 
# Build query
d="$f1:$ssl:$f4"
IFS=','
ips=$f3
 
# Call our master script to setup nginx reverse proxy / load balancer (LB) for given domain name
$_setup "$d" $ips

#2.1: Display an Error Message and Run Command

If $2 is not set display an error message for $2 parameter and run cp command on fly as follows:

#!/bin/bash
_file="$HOME/.input"
_message="Usage: chkfile commandname"
 
# Run another command (compact format)
_cmd="${2:? $_message $(cp $_file $HOME/.output)}"
 
$_cmd "$_file"

#3: Find Variable Length

You can easily find string length using the following syntax:

${#variableName}
echo ${#variableName}
len=${#var}

Here is a sample shell script to add a ftp user:

#!/bin/bash
# Usage : Add a ftp user
_fuser="$1"
_fpass="$2"
 
# die if username/password not provided
[ $# -ne 2 ] && { echo "Usage: addftpuser username password"; exit 1;}
 
# Get username length and make sure it always <= 8
[[ ${#_fuser} -ge 9 ]] && { echo "Error: Username should be maximum 8 characters in length. "; exit 2;}
 
# Check for existing user in /etc/passwd
/usr/bin/getent passwd "${_fuser}" &>/dev/null
 
# Check exit status
[ $? -eq 0 ] && { echo "Error: FTP username \"${_fuser}\" exists."; exit 3; }
 
# Add user
/sbin/useradd -s /sbin/nologin -m  "${_fuser}"
echo "${_fpass}" | /usr/bin/passwd "${_fuser}" --stdin

Each Linux or UNIX command returns a status when it terminates normally or abnormally. You can use command exit status in the shell script to display an error message or take some sort of action. In above example, if getent command is successful, it returns a code which tells the shell script to display an error message. 0 exit status means the command was successful without any errors. $? holds the return value set by the previously executed command.

#4: Remove Pattern (Front of $VAR)

The syntax is as follows:

${var#Pattern}
${var##Pattern}

You can strip $var as per given pattern from front of $var. In this example remove /etc/ part and get a filename only, enter:

f="/etc/resolv.conf"
echo ${f#/etc/}

The first syntax removes shortest part of pattern and the second syntax removes the longest part of the pattern. Consider the following example:

_version="20090128"
_url="http://dns.measurement-factory.com/tools/dnstop/src/dnstop-${_version}.tar.gz"

You just want to get filename i.e. dnstop-20090128.tar.gz, enter (try to remove shortest part of $_url) :

echo "${_url#*/}"

Sample outputs:

/dns.measurement-factory.com/tools/dnstop/src/dnstop-20090128.tar.gz

Now try using the longest part of the pattern syntax:

echo "${_url##*/}"

Sample outputs:

dnstop-20090128.tar.gz

This is also useful to get a script name without using /bin/basename:

#!/bin/bash
_self="${0##*/}"
echo "$_self is called"

Create a script called master.info as follows:

#!/bin/bash
# Purpose: Display jail info as per softlink
# Author: Vivek Gite
_j="[email protected]"
 
# find out script name
_self="${0##*/}"
 
[ "$VERBOSE" == "1" ] && echo "Called as $_self for \"$_j\" domain(s)"
 
for j in $_j
do
	export _DOMAIN_NAME=$j
	source functions.sh
	init_jail
        # call appropriate functions as per script-name / softlink
	case $_self in
		uploaddir.info) echo "Upload dir for $j: $(get_domain_upload_dir)" ;;
		tmpdir.info) echo "/tmp dir for $j: $(get_domain_tmp_dir)" ;;
                mem.info) echo "$j domain mem usage (php+lighttpd): $(get_domain_mem_info)" ;;
                cpu.info) echo "$j domain cpu usage (php+lighttpd): $(get_domain_cpu_info)" ;;
                user.info) echo "$j domain user and group info: $(get_domain_users_info)" ;;
                diskquota.info) echo "$j domain disk quota info (mysql+disk): $(get_domain_diskquota_info)" ;;
		*) warn "Usage: $_self"
	esac
done

Finally, create softlink as follows:
# ln -s master.info uploaddir.info
# ln -s master.info tmpdir.info
# ln -s master.info mem.info
....
..

You can now call script as follows:
# ./mem.info example.org
# ./cpu.info example.com example.net

#4.1: Remove Pattern (Back of $VAR)

The syntax is as follows:

${var%pattern}
${var%%pattern}

Exactly the same as above, except that it applies to the back of $var. In this example remove .tar.gz from $FILE, enter:

FILE="xcache-1.3.0.tar.gz"
echo ${FILE%.tar.gz}

Sample outputs:

xcache-1.3.0

Rename all *.perl files to *.pl using bash for loop as Apache web server is configured to only use .pl file and not .perl file names:

for p in /scripts/projects/.devl/perl/*.perl
do
	mv "$p" "${p%.perl}.pl"
done

You can combine all of them as follows to create a build scripts:

#!/bin/bash
# Usage: Build suhosin module for RHEL based servers
# Author: Vivek Gite
# ----
# Set default value for $2
VERSION="-${2:-0.9.31}"
URL="http://download.suhosin.org/suhosin${VERSION}.tgz"
vURL="http://download.suhosin.org/suhosin${VERSION}.tgz.sig"
 
# Get tar ball names
FILE="${URL##*/}"
vFILE="${vURL##*/}"
DLHOME="/opt"
SOFTWARE="suhosin"
 
# Remove .tgz and get dir name
DEST="${FILE%.tgz}"
 
# Download software
wget $URL -O "${DLHOME}/$FILE"
wget $vURL -O "${DLHOME}/$vFILE"
 
# Extract it
tar -zxvf $FILE
cd "$DEST"
 
# Build it and install it
phpize --clean && phpize && ./configure && make && read -p "Update/Install $SOFTWARE [Y/n] ? " answer
shopt -s nocasematch
[[ $answer =~ y|es  ]] && make install
shopt -u nocasematch

If you turn on nocasematch option, shell matches patterns in a case-insensitive fashion when performing matching while executing case or [[ conditional expression.

#5: Find And Replace

The syntax is as follows:

${varName/Pattern/Replacement}
${varName/word1/word2}
${os/Unix/Linux}

Find word unix and replace with linux, enter:

x="Use unix or die"
sed 's/unix/linux/' <<<$x

You can avoid using sed as follows:

echo "${x/unix/linux}"
out="${x/unix/linux}"
echo "${out}"

To replace all matches of pattern, enter :

out="${x//unix/linux}"

You can use this to rename or remove files on fly

y=/etc/resolv.conf
cp "${y}" "${y/.conf/.conf.bak}"

Here is another example:

         # RHEL php modules path
	_php_modules="/usr/lib64/php/modules"
	for i in $_php_modules/*
	do
		p="${i##*/}"                  ## Get module name
		ini="/etc/php.d/${p/so/ini}"  ## Get ini file by replacing .so with .ini extension
                # make sure file exists
		[ ! -f "$ini" ] && echo "$i php module exists but $ini file not found."
	done

The following function installs required modules in chrooted php-cgi process

install_php_modules(){
        # get jail name
	local n="${_chrootbase}/${d##/}"
	local p=""
	local ini=""
        # enable only ${_php_modules_enabled} php modules and delete other .ini files if exists in jail
	for i in $_php_modules/*
	do
		p="${i##*/}"
		ini="$n/etc/php.d/${p/so/ini}"
                # find out if module is enabled or not
		if [[ ${_php_modules_enabled} = *${p}*   ]]
		then
			[ "$VERBOSE" == "1" ] && echo " [+] Enabling php module $p"
			$_cp -f "$i" "$n/${_php_modules##/}"      ## install it
			copy_shared_libs "$i"                     ## get shared libs in jail too
		else
			[ -f "${ini}" ] && $_rm -f "${ini}"	  ## if old .ini exists in jail, just delete it
		fi
	done
}

#6: Substring Starting Character

The syntax is as follows:

${parameter:offset}
${parameter:offset:length}
${variable:position}
var=${string:position}

Expands to up to length characters of parameter starting at the character specified by offset.

base="/backup/nas"
file="/data.tar.gz"
#### strip extra slash from $file  ####
path="${base}/${file:1}"

Extract craft word only:

x="nixcraft.com"
echo ${x:3:5}"

To extract phone number, enter:

phone="022-124567887"
# strip std code
echo "${phone:4}"

Summary: String Manipulation and Expanding Variables

For your ready references here are all your handy bash parameter substitution operators. Try them all; enhance your scripting skills like a pro:

${parameter:-defaultValue} Get default shell variables value
${parameter:=defaultValue} Set default shell variables value
${parameter:?”Error Message”} Display an error message if parameter is not set
${#var} Find the length of the string
${var%pattern} Remove from shortest rear (end) pattern
${var%%pattern} Remove from longest rear (end) pattern
${var:num1:num2}Substring
${var#pattern} Remove from shortest front pattern
${var##pattern} Remove from longest front pattern
${var/pattern/string} Find and replace (only replace first occurrence)
${var//pattern/string} Find and replace all occurrences
References:

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

Share this on (or read 31 comments/add one below):

31 comment

    1. Backups are done ones a day. This post was restored from my local open office org cache file. There is not much I can do to restore all 12-13 comments including yours.

  1. What a great tutorial and reference. I can never remember the syntax when I need it. Will definitely be bookmarking this one.

          1. You’re right. Not sure where my brain was that day. One would I assume I tested it before commenting, but it doesn’t seem to do anything. No error or warning either. That’s bash for you. 😛

  2. Thanks for this excellent HowTo! One minor correction: sed ‘s/unix/linux’ <<<$x should be sed 's/unix/linux/g' <<<$x

    Otherwise you get an unterminated `s' command error.

  3. on the bash-shell-parameter-substitution-2.html page, _mkdir() example, fourth line you write:
    [ $# -eq 0 ] &shouldn’t this be
    [ $# -eq 0 ] &## [0—] is four digit octal permissions

    or equivalent to allow user to set permissions other than the default 0755 value?

  4. prior message was strangely truncated in places. Resubmit:

    on the bash-shell-parameter-substitution-2.html page, _mkdir() example, fourth line you write:

    [ $# -eq 0 ] &shouldn’t this be:

    [ $# -eq 0 ] &## [0—] is four digit octal permissions

    to inform user to set permissions other than the default 0755 value if desired?

  5. Last attempt: on the bash-shell-parameter-substitution-2.html page, _mkdir() example, 4th line:
    [ $# -eq 0 ] &shouldn’t this be:
    [ $# -eq 0 ] &## [0—] is octal permissions
    to inform user to set other than the default 0755 value if desired?

  6. Something interesting to note on the substring syntax is that you can address the offset backwards as well. I am not sure about how portable this syntax is, but I have found it very useful:

    echo “${VAR: -4}” # Prints the last 4 characters, note the space between : and – in this example.

    echo “${VAR:1:-1}” # The space is optional on the second offset, but not the first

    1. @khoan, yes, what did you expect? There is no command named ‘value’. Put echo in front and it should output ‘value’ unless $bam is already set.

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