String manipulation in bash

Update on 04/24/2016. see a post Shell parameter substitution

${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

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 manipulation below:

Let's create 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:

In [2]:

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

In [4]:

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

bar.txt.bar.txt

In [5]:

echo "${file_1/foo/bar}"

bar.txt.foo.txt

In [6]:

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

bar.txt.foo.txt

In [7]:

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

In [11]:

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

In [13]:

echo "${file_1:4:7}"

txt.foo

Finally, the length of the string:

In [14]:

echo "${#file_1}"

15