ReplacingStrings

Variables

If it's a variable, this can (and should) be done very simply with parameter expansion. Forking an external tool for string manipulation is extremely slow and unnecessary.

var='some string'; search=some; rep=another

# Bash
var=${var//"$search"/$rep}


# POSIX function

# usage: string_rep SEARCH REPL STRING
# replaces all instances of SEARCH with REPL in STRING
string_rep() {
  # initialize vars
  in=$3
  unset out

  # SEARCH must not be empty
  [ "$1" ] || return

  while true; do
    # break loop if SEARCH is no longer in "$in"
    case "$in" in
      *"$1"*) : ;;
      *) break;;
    esac

    # append everything in "$in", up to the first instance of SEARCH, and REP, to "$out"
    out=$out${in%%"$1"*}$2
    # remove everything up to and including the first instance of SEARCH from "$in"
    in=${in#*"$1"}
  done

  # append whatever is left in "$in" after the last instance of SEARCH to out, and print
  printf '%s%s\n' "$out" "$in"
}

var=$(string_rep "$search" "$rep" "$var")

# Note: POSIX does not have a way to localize variables. Most shells (even dash and 
# busybox), however, do. Feel free to localize the variables if your shell supports
# it. Even if it does not, if you call the function with var=$(string_rep ...), the
# function will be run in a subshell and any assignments it makes will not persist.

In the bash example, the quotes around "$search" prevent the contents of the variable to be treated as a shell pattern (also called a "glob"). Of course, if pattern matching is intended, do not include the quotes. If "$rep" were quoted, however, the quotes would be treated as literal.

Parameter expansions like this are discussed in more detail in Faq #100.

Streams

If it's a file or a stream, things get a little bit trickier. The standard tools available for this are sed or AWK (for streams), and ed (for files).

Of course, you could do it in bash itself, by combining the previous method with Faq #1:

search=foo; rep=bar

while IFS= read -r line; do
  printf '%s\n' "${line//"$search"/$rep}"
done < <(some_command)

some_command | while IFS= read -r line; do
  printf '%s\n' "${line//"$search"/$rep}"
done

If you want to do more processing than just a simple search/replace, this may be the best option. Note that the last example runs the loop in a subshell. See Faq #24 for more information on that.

Another option would, of course, be sed:

# replaces all instances of "search" with "replace" in the output of "some_command"
some_command | sed 's/search/replace/g'

sed uses regular expressions. Unlike the bash, "search" and "replace" would have to be rigorously escaped in order to treat the values as literal strings. This is very impractical, and attempting to do so will make your code extremely prone to bugs. Embedding shell variables in sed is never a good idea.

You may notice, however, that the bash loop above is very slow for large data sets. So how do we find something faster, that can replace literal strings? Well, you could use AWK. The following function replaces all instances of STR with REP, reading from stdin and writing to stdout.

# usage: gsub_literal STR REP
# replaces all instances of STR with REP. reads from stdin and writes to stdout.
gsub_literal() {
  # STR cannot be empty
  [[ $1 ]] || return

  # string manip needed to escape '\'s, so awk doesn't expand '\n' and such
  awk -v str="${1//\\/\\\\}" -v rep="${2//\\/\\\\}" '
    # get the length of the search string
    BEGIN {
      len = length(str);
    }

    {
      # empty the output string
      out = "";

      # continue looping while the search string is in the line
      while (i = index($0, str)) {
        # append everything up to the search string, and the replacement string
        out = out substr($0, 1, i-1) rep;

        # remove everything up to and including the first instance of the
        # search string from the line
        $0 = substr($0, i + len);
      }

      # append whatever is left
      out = out $0;

      print out;
    }
  '
}

some_command | gsub_literal "$search" "$rep"


# condensed as a one-liner:
some_command | awk -v s="${search//\\/\\\\}" -v r="${rep//\\/\\\\}" 'BEGIN {l=length(s)} {o="";while (i=index($0, s)) {o=o substr($0,1,i-1) r; $0=substr($0,i+l)} print o $0}'

CategoryShell