Replacing all occurrences of one string with another in all files in the current directory:

gistfile1.txt

These are for cases where you know that the directory contains only regular files and that you want to process all non-hidden files. If that is not the case, use the approaches in 2.

All sed solutions in this answer assume GNU sed. If using FreeBSD or macOS, replace -i with -i ''. Also note that the use of the -i switch with any version of sed has certain filesystem security implications and is inadvisable in any script which you plan to distribute in any way.

Non recursive, files in this directory only:

 sed -i -- 's/foo/bar/g' *
 perl -i -pe 's/foo/bar/g' ./* 
(the perl one will fail for file names ending in | or space)).

Recursive, regular files (including hidden ones) in this and all subdirectories

 find . -type f -exec sed -i 's/foo/bar/g' {} +
If you are using zsh:

 sed -i -- 's/foo/bar/g' **/*(D.)
(may fail if the list is too big, see zargs to work around).

Bash can't check directly for regular files, a loop is needed (braces avoid setting the options globally):

 ( shopt -s globstar dotglob;
     for file in **; do
         if [[ -f $file ]] && [[ -w $file ]]; then
             sed -i -- 's/foo/bar/g' "$file"
         fi
     done
 )
The files are selected when they are actual files (-f) and they are writable (-w).

2. Replace only if the file name matches another string / has a specific extension / is of a certain type etc:
Non-recursive, files in this directory only:

sed -i -- 's/foo/bar/g' *baz*    ## all files whose name contains baz
sed -i -- 's/foo/bar/g' *.baz    ## files ending in .baz
Recursive, regular files in this and all subdirectories

find . -type f -name "*baz*" -exec sed -i 's/foo/bar/g' {} +
If you are using bash (braces avoid setting the options globally):

( shopt -s globstar dotglob
    sed -i -- 's/foo/bar/g' **baz*
    sed -i -- 's/foo/bar/g' **.baz
)
If you are using zsh:

sed -i -- 's/foo/bar/g' **/*baz*(D.)
sed -i -- 's/foo/bar/g' **/*.baz(D.)
The -- serves to tell sed that no more flags will be given in the command line. This is useful to protect against file names starting with -.

If a file is of a certain type, for example, executable (see man find for more options):

find . -type f -executable -exec sed -i 's/foo/bar/g' {} +
zsh:

    sed -i -- 's/foo/bar/g' **/*(D*)
3. Replace only if the string is found in a certain context
Replace foo with bar only if there is a baz later on the same line:

 sed -i 's/foo\(.*baz\)/bar\1/' file
In sed, using \( \) saves whatever is in the parentheses and you can then access it with \1. There are many variations of this theme, to learn more about such regular expressions, see here.

Replace foo with bar only if foo is found on the 3d column (field) of the input file (assuming whitespace-separated fields):

 gawk -i inplace '{gsub(/foo/,"baz",$3); print}' file
(needs gawk 4.1.0 or newer).

For a different field just use $N where N is the number of the field of interest. For a different field separator (: in this example) use:

 gawk -i inplace -F':' '{gsub(/foo/,"baz",$3);print}' file
Another solution using perl:

    perl -i -ane '$F[2]=~s/foo/baz/g; $" = " "; print "@F\n"' foo 
NOTE: both the awk and perl solutions will affect spacing in the file (remove the leading and trailing blanks, and convert sequences of blanks to one space character in those lines that match). For a different field, use $F[N-1] where N is the field number you want and for a different field separator use (the $"=":" sets the output field separator to :):

    perl -i -F':' -ane '$F[2]=~s/foo/baz/g; $"=":";print "@F"' foo 
Replace foo with bar only on the 4th line:

 sed -i '4s/foo/bar/g' file
 gawk -i inplace 'NR==4{gsub(/foo/,"baz")};1' file
 perl -i -pe 's/foo/bar/g if $.==4' file
4. Multiple replace operations: replace with different strings
You can combine sed commands:

 sed -i 's/foo/bar/g; s/baz/zab/g; s/Alice/Joan/g' file
Be aware that order matters (sed 's/foo/bar/g; s/bar/baz/g' will substitute foo with baz).

or Perl commands

 perl -i -pe 's/foo/bar/g; s/baz/zab/g; s/Alice/Joan/g' file
If you have a large number of patterns, it is easier to save your patterns and their replacements in a sed script file:

 #! /usr/bin/sed -f
 s/foo/bar/g
 s/baz/zab/g
Or, if you have too many pattern pairs for the above to be feasible, you can read pattern pairs from a file (two space separated patterns, $pattern and $replacement, per line):

 while read -r pattern replacement; do   
     sed -i "s/$pattern/$replacement/" file
 done < patterns.txt
That will be quite slow for long lists of patterns and large data files so you might want to read the patterns and create a sed script from them instead. The following assumes a <<!>space<!>> delimiter separates a list of MATCH<<!>space<!>>REPLACE pairs occurring one-per-line in the file patterns.txt :

 sed 's| *\([^ ]*\) *\([^ ]*\).*|s/\1/\2/g|' <patterns.txt |
 sed -f- ./editfile >outfile
The above format is largely arbitrary and, for example, doesn't allow for a <<!>space<!>> in either of MATCH or REPLACE. The method is very general though: basically, if you can create an output stream which looks like a sed script, then you can source that stream as a sed script by specifying sed's script file as -stdin.

You can combine and concatenate multiple scripts in similar fashion:

 SOME_PIPELINE |
 sed -e'#some expression script'  \
     -f./script_file -f-          \
     -e'#more inline expressions' \
 ./actual_edit_file >./outfile
A POSIX sed will concatenate all scripts into one in the order they appear on the command-line. None of these need end in a \newline.

grep can work the same way:

 sed -e'#generate a pattern list' <in |
 grep -f- ./grepped_file
When working with fixed-strings as patterns, it is good practice to escape regular expression metacharacters. You can do this rather easily:

 sed 's/[]$&^*\./[]/\\&/g
      s| *\([^ ]*\) *\([^ ]*\).*|s/\1/\2/g|
 ' <patterns.txt |
 sed -f- ./editfile >outfile
5. Multiple replace operations: replace multiple patterns with the same string
Replace any of foo, bar or baz with foobar

 sed -Ei 's/foo|bar|baz/foobar/g' file
or

 perl -i -pe 's/foo|bar|baz/foobar/g' file
6. Replace File paths in multiple files
Another use case of using different delimiter:

 sed -i 's|path/to/foo|path/to/bar|g' *
or

 perl -i -pe 's|path/to/foo|path/to/bar|g' ./*

https://unix.stackexchange.com/questions/112023/how-can-i-replace-a-string-in-a-files