grep files from list
Solution 1
You seem to be grepping the list of filenames, not the files themselves. <(cat files.txt)
just lists the files. Try <(cat $(cat files.txt))
to actually concatenate them and search them as a single stream, or
grep -i 'foo' $(cat files.txt)
to give grep all the files.
However, if there are too many files on the list, you may have problems with number of arguments. In that case I'd just write
while read filename; do grep -Hi 'foo' "$filename"; done < files.txt
Solution 2
xargs grep -i -- foo /dev/null < files.txt
assuming files are blank or newline delimited (where quotes or backslashes can be used to escape those separators). With GNU xargs
you can specify the delimiter with -d
(which then disables the quoting handling though).
(unset -v IFS; set -f; grep -i -- foo $(cat files.txt))
assuming files are space, tab or newline separated (no way to escape those though you can choose a different separator by assigning it to IFS
). That one will fail if the file list is too big on most systems.
Those also assume that none of the files are called -
.
Solution 3
To read a list of file names from stdin you can use xargs
. E.g.,
cat files.txt | xargs -d'\n' grep -i -- 'foo'
By default, xargs
reads items from the standard input, delimited by blanks. The -d'\n'
tells it to use newline as the argument delimiter, so it can handle file names containing blanks. (As Stéphane Chazelas points out, that's a GNU extension). However, it won't cope with file names containing newlines; we'd need a slightly more complicated approach to handle those.
FWIW, this approach is somewhat faster than a while read
loop, as bash's read
command is very slow - it reads its data character by character, whereas xargs
reads its input more efficiently. Also, xargs
only invokes the grep
command as many times as it needs to, with each invocation receiving multiple file names, and that's more efficient than invoking grep
individually for each file name.
See the xargs man page and the xargs info page for further details.
Solution 4
xargs
can read items from a file (like your files.txt
list) with it's option:
--arg-file=file
-a file
Read items from file instead of standard input. If you use this
option, stdin remains unchanged when commands are run. Other‐
wise, stdin is redirected from /dev/null.
So this should work too:
xargs -a files.txt grep -i 'foo'
or for spaces in filenames
xargs -d'\n' -a files.txt grep -i 'foo'
xargs -I{} -a files.txt grep -i 'foo' {}
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dotancohen
Updated on September 18, 2022Comments
-
dotancohen almost 2 years
I am trying to run grep against a list of a few hundred files:
$ head -n 3 <(cat files.txt) admin.php ajax/accept.php ajax/add_note.php
However, even though I am grepping for a string that I know is found in the files, the following does not search the files:
$ grep -i 'foo' <(cat files.txt) $ grep -i 'foo' admin.php The foo was found
I am familiar with the
-f
flag which will read the patterns from a file. But how to read the input files?I had considered the horrible workaround of copying the files to a temporary directory as
cp
seems to support the<(cat files.txt)
format, and from there grepping the files. Shirley there is a better way. -
dotancohen over 9 yearsThank you! I did not realize that
while
could receive the lines of file.txt as such. -
Stéphane Chazelas over 9 yearsYou'll want to disable the glob part of that split+glob operator here (unless the shell is zsh).
-
Stéphane Chazelas over 9 yearsTo read a (text) line, the syntax is
IFS= read -r filename
,read filename
is something else. -
Stéphane Chazelas over 9 yearsNote that
-H
is a GNU extension. You're missing some--
. -
PM 2Ring over 9 years@dotancohen: For more info on
read -r
see unix.stackexchange.com/q/18886/88378 and unix.stackexchange.com/q/87301/88378 -
jimmij over 9 yearsIt is better/faster to use
$(< file)
instead of$(cat file)
, at least inbash
andzsh
.