Prefix and suffix strings to each output line from command

Question

I have ran into a problem trying to write a Bash script. When grep outputs, it returns (usually) many lines. I would like to prefix and suffix a string to each of these output lines.

I would also like to note that I'm piping ls into grep, like:

ls | grep

Answer 1

With sed:

ls | grep txt | sed 's/.*/prefix&suffix/'

Answer 2

With sed:

ls | grep pattern | sed -e 's/^/prefix/' -e 's/$/suffix/'

But note, this suffers from problems with filenames with line feeds, and all the assorted problems of parsing ls which is generally a bad idea.

With perl

perl -e 'print "prefix".$_."suffix\n" for grep { m/somepattern/} glob "*"'

Note - perl grep can take a pattern - like the grep command, but you can also do things like apply file tests.

E.g.

grep {-d} glob "*" #filters directories.
grep { (stat)[9] > time() - 60 } glob "*" #reads file mtime and compares. 

Within grep the default iterator $_ is set to the current element value, so you can apply sed/grep style regex, or perform a variety of tasks based on $_.

Answer 3

In this case GNU find lets you do all these things in one go, eliminating pipes and potentially troublesome parsing of ls:

find . -maxdepth 1 -name '[^.]*' \
  -regextype posix-extended -regex "MYPATTERN" \
  -printf 'SOMEPREFIX %f SOMESUFFIX\n'

(find is not a good way to arbitrarily modify the output of other commands, of course!)

Some notes:

• -maxdepth 1 and -name [^.]* make name matching work the same as a plain ls (or ls .). You can use any shell-style glob, but note that "*" will match a leading "." in a name^† unlike bash, so [^.]* means anything that doesn't have a leading "."
• MYPATTERN is a proper POSIX ERE (default type is Emacs, see here), but it must match the entire filename (including the ./ prefix) so use something like .*thing.* instead of just thing
• you can probably just use one of -name/-regex instead of both (e.g. -regex "[^.].MYPATTERN."
• -printf supports lots of things, %n is the unadorned file or directory name
• beware find (contrary to ls) doesn't sort the list of file names.
• it assumes the file names constitute valid text in the current locale.

^† (this can depend on your version of find though, check the "STANDARDS CONFORMANCE" section of your man page)

As a possible alternative with no external programs required, bash has compgen which, among other things, expands globs, equivalent to an ls with no options:

compgen -P "someprefix>" -S "<somesuffix" -G "pattern"

This handles filenames with whitespace, including newlines. compgen -G "*" should provide the same output as a plain ls (but note that ls * is a different thing entirely). You'll still need grep, and this may or may not solve the problem, but it's worth a mention.

Answer 4

Maybe you can just use globs here. With the extglob option enabled (which enables ksh88 globs), bash globs are functionally equivalent to regexps so you don't need grep:

printf 'prefix%ssuffix\n' *foo*

For all the filenames that contain foo (equivalent to using grep foo with the extra benefit that it works properly if filenames contain newline characters).

shopt -s extglob
printf 'prefix%ssuffix\n' foo+([[:digit:]]).jpg

(as equivalent to grep -E '^foo[[:digit:]]+\.jpg$')

Answer 5

With awk

ls | grep | awk '$0="prefix"$0"suffix"'

"prefix" and "suffix" are string literals, so replace the text with whatever you want.

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Sources: https://stackoverflow.com/a/2099492/2089675

Answer 6

sed and find solutions are challenged if your prefix/suffix include regex key characters or if want to pass an environment variable (instead of an immediate value), xargs+echo might be more appropriate here.

With environment variable:

ls | grep p | xargs -I % sh -c "echo $PATH/%"

With an inline:

ls | grep p | xargs -I % sh -c "echo `pwd`/%"