In bash shell, when you use a dollar sign followed by a variable name, shell expands the variable with its value. This feature of shell is called parameter expansion.
But parameter expansion has numerous other forms which allow you to expand a parameter and modify the value or substitute other values in the expansion process. In this article, let us review how to use the parameter expansion concept for string manipulation operations.
This article is part of the on-going bash tutorial series. Refer to our earlier article on bash { } expansion.
1. Identify String Length inside Bash Shell Script
${#string}
The above format is used to get the length of the given bash variable.
$ cat len.sh
#! /bin/bash
var="Welcome to the geekstuff"
echo ${#var}
$ ./len.sh
24
To understand more about bash variables, read 6 Practical Bash Global and Local Variable Examples.
2. Extract a Substring from a Variable inside Bash Shell Script
Bash provides a way to extract a substring from a string. The following example expains how to parse n characters starting from a particular position.
${string:position}
Extract substring from $string at $position
${string:position:length}
Extract $length of characters substring from $string starting from $position. In the below example, first echo statement returns the substring starting from 15th position. Second echo statement returns the 4 characters starting from 15th position. Length must be the number greater than or equal to zero.
$ cat substr.sh
#! /bin/bash
var="Welcome to the geekstuff"
echo ${var:15}
echo ${var:15:4}
$ ./substr.sh
geekstuff
geek
Also, refer to our earlier article to understand more about $*, $@, $#, $$, $!, $?, $-, $_ bash special parameters.
3. Shortest Substring Match
Following syntax deletes the shortest match of $substring from front of $string
${string#substring}
Following syntax deletes the shortest match of $substring from back of $string
${string%substring}
Following sample shell script explains the above two shortest substring match concepts.
$ cat shortest.sh
#! /bin/bash
filename="bash.string.txt"
echo ${filename#*.}
echo ${filename%.*}
$ ./shortest.sh
After deletion of shortest match from front: string.txt
After deletion of shortest match from back: bash.string
In the first echo statement substring ‘*.’ matches the characters and a dot, and # strips from the front of the string, so it strips the substring “bash.” from the variable called filename. In second echo statement substring ‘.*’ matches the substring starts with dot, and % strips from back of the string, so it deletes the substring ‘.txt’
4. Longest Substring Match
Following syntax deletes the longest match of $substring from front of $string
${string##substring}
Following syntax deletes the longest match of $substring from back of $string
${string%%substring}
Following sample shell script explains the above two longest substring match concepts.
$ cat longest.sh
#! /bin/bash
filename="bash.string.txt"
echo "After deletion of longest match from front:" ${filename##*.}
echo "After deletion of longest match from back:" ${filename%%.*}
$ ./longest.sh
After deletion of longest match from front: txt
After deletion of longest match from back: bash
In the above example, ##*. strips longest match for ‘*.’ which matches “bash.string.” so after striping this, it prints the remaining txt. And %%.* strips the longest match for .* from back which matches “.string.txt”, after striping it returns “bash”.
5. Find and Replace String Values inside Bash Shell Script
Replace only first match
${string/pattern/replacement}
It matches the pattern in the variable $string, and replace only the first match of the pattern with the replacement.
$ cat firstmatch.sh
#! /bin/bash
filename="bash.string.txt"
echo "After Replacement:" ${filename/str*./operations.}
$ ./firstmatch.sh
After Replacement: bash.operations.txt
Replace all the matches
${string//pattern/replacement}
It replaces all the matches of pattern with replacement.
$ cat allmatch.sh
#! /bin/bash
filename="Path of the bash is /bin/bash"
echo "After Replacement:" ${filename//bash/sh}
$ ./allmatch.sh
After Replacement: Path of the sh is /bin/sh
Taking about find and replace, refer to our earlier articles – sed substitute examples and Vim find and replace.
Replace beginning and end
${string/#pattern/replacement}
Following syntax replaces with the replacement string, only when the pattern matches beginning of the $string.
${string/%pattern/replacement}
Following syntax replaces with the replacement string, only when the pattern matches at the end of the given $string.
$ cat posmatch.sh
#! /bin/bash
filename="/root/admin/monitoring/process.sh"
echo "Replaced at the beginning:" ${filename/#\/root/\/tmp}
echo "Replaced at the end": ${filename/%.*/.ksh}
$ ./posmatch.sh
Replaced at the beginning: /tmp/admin/monitoring/process.sh
Replaced at the end: /root/admin/monitoring/process.ksh
Recommended Reading
Bash 101 Hacks, by Ramesh Natarajan. I spend most of my time on Linux environment. So, naturally I’m a huge fan of Bash command line and shell scripting. 15 years back, when I was working on different flavors of *nix, I used to write lot of code on C shell and Korn shell. Later years, when I started working on Linux as system administrator, I pretty much automated every possible task using Bash shell scripting. Based on my Bash experience, I’ve written Bash 101 Hacks eBook that contains 101 practical examples on both Bash command line and shell scripting. If you’ve been thinking about mastering Bash, do yourself a favor and read this book, which will help you take control of your Bash command line and shell scripting.
My name is Ramesh Natarajan. I will be posting instruction guides, how-to, troubleshooting tips and tricks on Linux, database, hardware, security and web. My focus is to write articles that will either teach you or help you resolve a problem. Read more about
Comments on this entry are closed.
This is incredibly useful, thanks! You didn’t close the braces at “Replace beginning and end”, and in Example 3 the echo doesn’t contain “After deletion of shortest match from front/back:”.
great! thanks a lot for all theses tricks and explanations !
This is really helpful for me. Great!
Thanks man
If you use bash 4.x you can source the oobash. A string lib written in bash with oo-style:
http://sourceforge.net/projects/oobash/
String is the constructor function:
String a abcda
a.indexOf a
0
a.lastIndexOf a
4
a.indexOf da
3
There are many “methods” more to work with strings in your scripts:
-base64Decode -base64Encode -capitalize -center
-charAt -concat -contains -count
-endsWith -equals -equalsIgnoreCase -reverse
-hashCode -indexOf -isAlnum -isAlpha
-isAscii -isDigit -isEmpty -isHexDigit
-isLowerCase -isSpace -isPrintable -isUpperCase
-isVisible -lastIndexOf -length -matches
-replaceAll -replaceFirst -startsWith -substring
-swapCase -toLowerCase -toString -toUpperCase
-trim -zfill
Thanks a lot for script!
Very Good Article such that I saved its URL for future refrences if I need & Forget.
Thanks a lot for scripts saved my day
Thanks a lot !!
good stuff…
please guide me – how to (change/insert/delete) any character from a string by indicating the position
thank you
Hi
How to do the following
string1=”something$variable1something$variable1something”
variable1=”good”
#replacement command here
echo $string1; #should print “somethinggoodsomethinggood”
Appreciated
I have a querry………. please help!!!
suppose there is a string “hello 14 all -23 I am here”
Now from this string I want to extarct 14 and -23. How can I do so??????
Hi Ramesh, this guide save me from hell, thanks alot!
Thank you very much! You presented the simplest and elegant technique.
Excelent information.
Thanks a lot!
In the section “Replace beginning and end”, you are missing ‘}’ from the examples.
@Paul,
Thanks for catching the missing }. It is fixed now.
Thanks much for the useful and easy to follow examples. I really appreciate it!
Thanks for help …
Verry using data …
Hi my Guru,
./mail1@domain.com-examinations1_01_01-19.59.50.zip
./mail2@domain.com-examinations2_02_02-20.12.13.zip
I want to recursively mkdir first before unzip those zip files with the following folder structure:
/mail1@domain.com-examinations1/01/01-19.59.50
/mail2@domain.com-examinations2/02/02-20.12.13
I found the problem is how to replace each character “_” with character “/” in order to mkdir recursively and then unzip them to the created folder structure. Could you please help me?
Is there a way I can pull a substring out of a variable using these methods.
For example
var=host.domain.com
echo var and just retrieve “domain” from string.
Obviously I can do this with awk or cut… But just curious if pure bash expansion can do this.
One thing I noticed is that you put all the commands in a file, which leads newbies to think that they can’t be run directly from the $ prompt.
Thanks, nice tips