String manipulation is very useful in bash scripts; especially when processing a lot of files (in a for loop or xargs) with different names, and you want to name your output of each file with part of the input file name. command file part_file.result
I am going to demonstrate string maniputlation below:

Let's creat a variable file and print it out:



In [1]:

    
file=foo.txt
echo "$file"









    



foo.txt

I want to change the suffix from txt to pdf. one of the commonly known ways is to use the basename built-in function:



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echo "$(basename $file .txt).pdf"









    



foo.pdf

However, there are other built-in ways to get the same task done
references here

find and replace
Replace only first match:
${string/pattern/replacement}
Replace all the matches:
${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 $string. `${string/%pattern/replacement}`

For more complex replacement, use sed. see my previous blog post here



In [3]:

    
echo "${file/txt/pdf}"









    



foo.pdf



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# a more complex exmaple
file_1=foo.txt.foo.txt
echo "${file_1//foo/bar}"









    



bar.txt.bar.txt



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echo "${file_1/foo/bar}"









    



bar.txt.foo.txt



In [6]:

    
echo "${file_1/#foo/bar}"









    



bar.txt.foo.txt



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echo "${file_1/%txt/pdf}"









    



foo.txt.foo.pdf

${string%substring} will delete the shortest match of substring from back
${string%%substring} will delete the longest match of substring from back



In [8]:

    
echo "${file_1%txt*}pdf"









    



foo.txt.foo.pdf



In [9]:

    
echo "${file_1%%txt*}pdf"









    



foo.pdf

{string#substring} will delete the shortest match of substring from the begining
{string##substring} will delete the longest match of substring from the begining



In [10]:

    
echo "bar${file_1#foo*}"









    



bar.txt.foo.txt



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echo "bar${file_1##foo*}.pdf"









    



bar.pdf

string slicing
${string:position}
Extract substring from $string at `$position`

${string:position:length}
Extract $length of characters substring from $string starting from `$position`



In [12]:

    
echo "${file_1:4}"









    



txt.foo.txt



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echo "${file_1:4:7}"









    



txt.foo

Finally, the length of the string:



In [14]:

    
echo "${#file_1}"