BASH - Strings - Find & Replace

var="abcdabe"
 
echo ${var/ab/fg}

returns: 

fgcdabe

NOTE: Only the first match is changed.

To change all occurrences use: 

var="abcdabe"
 
echo ${var//ab/fg}

returns: 

fgcdfge

Changing contents within a file

contents=$(< infile.txt)
$ echo "${contents/$old/$new}"

NOTE: This reads the file into a Bash variable and uses parameter expansion.

  • infile.txt: The input file here is named infile.txt.
  • $old: The string to replace.
  • $new: The replacement string.

To change the file in-place: 

echo "${contents/$old/$new}" > infile.tmp && mv infile.tmp infile.txt