Bash Shell Loop Over Set of Files

by on May 11, 2008 · 37 comments· LAST UPDATED June 29, 2010

in , ,

How do I run shell loop over set of files stored in a current directory or specified directory?

You can use for loop easily over a set of shell file under bash or any other UNIX shell using wild card character.

Syntax

The general syntax is as follows:

 
for f in file1 file2 file3 file5
do
 echo "Processing $f"
 # do something on $f
done

You can also use shell variables:

 
FILES="file1
/path/to/file2
/etc/resolv.conf"
for f in $FILES
do
	echo "Processing $f"
done
 

You can loop through all files such as *.c, enter:

$ for f in *.c; do echo "Processing $f file.."; done

Sample Shell Script To Loop Through All Files

#!/bin/bash
FILES=/path/to/*
for f in $FILES
do
  echo "Processing $f file..."
  # take action on each file. $f store current file name
  cat $f
done

Filename Expansion

You can do filename expansion in loop such as work on all pdf files in current directory:

 
for f in *.pdf
do
	echo "Removing password for pdf file - $f"
done
 

However, there is one problem with the above syntax. If there are no pdf files in current directory it will expand to *.pdf (i.e. f will be set to *.pdf"). To avoid this problem add the following statement before the for loop:

#!/bin/bash
# Usage: remove all utility bills pdf file password 
shopt -s nullglob
for f in *.pdf
do
	echo "Removing password for pdf file - $f"
        pdftk "$f" output "output.$f" user_pw "YOURPASSWORD-HERE"
done
 

Using A Shell Variable And While Loop

You can read list of files from a text file. For example, create a text file called /tmp/data.txt as follows:

file1
file2
file3

Now you can use the while loop as follows to read and process each by one by one:

#!/bin/bash
        while IFS= read -r file
        do
                [ -f "$file" ] && rm -f "$file"
        done < "/tmp/data.txt"
 

Here is another example which removes all unwanted files from chrooted lighttpd / nginx or Apache webserver:

 
#!/bin/bash
_LIGHTTPD_ETC_DEL_CHROOT_FILES="/usr/local/nixcraft/conf/apache/secure/db/dir.etc.list"
secureEtcDir(){
        local d="$1"
        local _d="/jails/apache/$d/etc"
        local __d=""
        [ -f "$_LIGHTTPD_ETC_DEL_CHROOT_FILES" ] || { echo "Warning: $_LIGHTTPD_ETC_DEL_CHROOT_FILES file not found. Cannot secure files in jail etc directory."; return; }
        echo "* Cleaning etc FILES at: \"$_d\" ..."
        while IFS= read -r file
        do
                __d="$_d/$file"
                [ -f "$__d" ] && rm -f "$__d"
        done < "$_LIGHTTPD_ETC_DEL_CHROOT_FILES"
}
 
secureEtcDir "nixcraft.net.in"

Processing Command Line Arguments

 
#!/bin/bash
# make sure you always put $f in double quotes to avoid any nasty surprises i.e. "$f"
for f in $*
do
  echo "Processing $f file..."
  # rm "$f"
done
 

OR

 
#!/bin/bash
# make sure you always put $f in double quotes to avoid any nasty surprises i.e. "$f"
for f in $@
do
  echo "Processing $f file..."
  # rm "$f"
done
 

Please note that $@ expanded as "$1" "$2" "$3" ... "$n" and $* expanded as "$1y$2y$3y...$n", where y is the value of IFS variable i.e. "$*" is one long string and $IFS act as an separator or token delimiters.
The following example use shell variables to store actual path names and then files are processed using the for loop:

#!/bin/bash
_base="/jail/.conf"
_dfiles="${base}/nginx/etc/conf/*.conf"
 
for f in $_dfiles
do
        lb2file="/tmp/${f##*/}.$$"   #tmp file
        sed 's/Load_Balancer-1/Load_Balancer-2/' "$f" > "${lb2file}"   # update signature 
        scp "${lb2file}" nginx@lb2.nixcraft.net.in:${f}   # scp updated file to lb2
        rm -f "${lb2file}"
done
 

Updated for accuracy!

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{ 37 comments… read them below or add one }

1 DAY May 11, 2008 at 12:35 pm

The scripts are wrong, use `$FILES` (without quotes) in the loop instead of `”$FILES”`.

Reply

2 nixCraft May 11, 2008 at 1:57 pm

DAY,

I don’t think so it is wrong. Do you have any problem running script?

Reply

3 DAY May 11, 2008 at 3:14 pm

Yes, the script does not work for me. Both the one directly typed in the cmd line and the one from a script file. They output of the first one is:

Processing *.c file..

The output of the second is:

Processing * file…

In fact, I just start learn the shell script. Here is another one which can do the same job:

#!/bin/sh
for f in `ls`
do
echo “Processing $f file …”
done

Reply

4 Brock Noland May 11, 2008 at 7:36 pm

Hmm…not sure why you wouldn’t use

for file in *

or

for file in *.c

Reply

5 nixCraft May 11, 2008 at 8:54 pm

DAY,

Hmm, it should work (I hope you have *.c files in current directory) or try as suggested by brock.

Reply

6 DAY May 12, 2008 at 2:47 am

Thanks, vivek and Brock. Replacing “$FILES” with * or *.c works as expected. But I am just curious why the original one does not work for me. It seems that the all items following `in’ should not be enclosed in a pair of quotes, otherwise, all of them will be interpreted as one item, the string. I did try the following example, the output is as what I expected:

for f in "hello world"
do
echo "Processing $f file..."
# take action on each file. $f store current file name
done

Reply

7 DAY May 12, 2008 at 4:13 am

OK, read sth. in the bash man. page.

Quoting is used to remove the special meaning of certain characters or words to the shell. Quoting can be used to disable special treatment for special characters, to prevent reserved words from being recognized as such, and to prevent parameter expansion.

I think that’s why the original script doesn’t work. “$FILES” is treated as “*”, which does disable special treatment for the special char `*’. It’s similar when you type `echo “*”`. `echo’ would not print out everything in the `pwd`. Instead, it simply prints out `*’.

Reply

8 Jeff Schroeder May 12, 2008 at 5:08 pm

Actually… if $FILES + the contents of /proc/$pid/environ are together > the output of “getconf ARG_MAX” this will fail.

The proper way to do this that always works is in the “useless use of cat awards” page:
http://partmaps.org/era/unix/award.html#backticks

The for is easier to read, but it is really annoying when your scripts fail with the dreaded “argument list too long” errors.

Reply

9 Jeff Schroeder May 12, 2008 at 5:10 pm

Forgot to share a little bashism that makes it easy to determine a good guess of the ARG_MAX:
MAXARGS=$(( $(getconf ARG_MAX) – $(env | wc -c) ))

Reply

10 David Thompson May 16, 2008 at 4:46 am

I use loops like this a lot,

for x in * ; do
test -f “$x” || continue
COMMAND “$x”
done

helps to easily ignore subdirectories

Reply

11 Baz June 8, 2008 at 12:12 am

Double quotes disable the special meaning of most enclosed characters. They do not disable the interpretation of variables with a leading $. To do this yopu need single quotes.

FILES=”*” is wrong unless you want the value of $FILES to be *. The same is true of “*.c”. Lose the quotes to get what you want.

I have always just used –

for F in *
do
… etc

for F in `ls`
is OK except that
for F in ls -1 (one)
is better, but both are more cumbersome and less elegant that
for F in * (or *.c and so on)

Reply

12 Chris December 11, 2008 at 1:00 am

> FILES=”*” is wrong unless you want the value of $FILES to be *. The same is true of “*.c”. Lose the quotes to get what you want.

I dont think his wrong. He just gave those of us who are new to Bash scripting a placeholder for other commands or values. In fairness, the script did what it said it would do. Thanks for explaining the difference with using quotes and doing away with them.

For others reading this. Dont just take our words for it. Use the script – test it for yourself. Play with it. Thanks and congratulations to nixcraft for sharing.

Peace!

Reply

13 Foo February 11, 2009 at 9:47 pm

FILES=*; for f in $FILES; do… is WRONG.
for f in `ls`; do… is even WORSE.
Both break with filenames including whitespaces, newlines etc.

Since this is about bash use array if you want files in variables:
files=(*.c)
for f in “${files[@]}”; do cmd “$f”; done

Or just use glob:
for f in *.c; do cmd “$f”; done

Reply

14 Michael Smith February 11, 2009 at 10:00 pm

I was initially a little confused by the thread. It’s not useful to assign * to a variable if you only intend to use it for a loop. Furthermore, as others have stated, putting quotes around the variable name prevent the glob, in this case *, from expanding.
* Vivek is not correct that $FILES should be quoted.
* DAY’s initial response that $FILES should be unquoted is not wrong, but using the variable at all is not useful.
* DAY’s second idea of looping over the output of `ls` is a very common mistake. It’s wrong because of wordsplitting.
* Brock Noland’s instinct to use for file in *.c... is spot-on.
* Jeff Schroeder is right to avoid ARG_MAX in general, but it only applies when you call exec*() via the kernel. Since for is a shell builtin, ARG_MAX doesn’t apply here.
* David Thompson and Baz’s comments are OK, but to Baz I would reiterate to avoid using the ls command for anything except human-readable output.
* As for Chris’ comment: FILES="*" and FILES=* are equivalent since sh-compliant shells don’t expand globs during variable assignment.

Reply

15 rduke15 April 12, 2009 at 2:40 pm

One curious problem. If there are NO files matching the glob, you get this:

$ for file in *.jpg; do echo " the file variable is now '$file' " ; done

the file variable is now ‘*.jpg’

I would have expected that the contents of the for loop would not be executed at all, since there was no jpg file.

Reply

16 rduke15 April 12, 2009 at 2:53 pm

Found the problem with my previous example. The nullglob shell option needs to be set:
shopt -s nullglob; for file in *.jpg; do echo " the file variable is now '$file' " ; done

produces no output, as expected. As opposed to:

shopt -u nullglob; for file in *.jpg; do echo " the file variable is now '$file' " ; done
the file variable is now '*.jpg'

Reply

17 TheBonsai May 5, 2009 at 4:57 am

@Jeff Schroeder:


The for is easier to read, but it is really annoying when your scripts fail with the dreaded “argument list too long” errors.

This won’t happen on a for-loop statement, since no exec() is done for the loop itself.

Reply

18 gurpur December 17, 2009 at 3:29 am

I need some in writing a script that will delete a specific line from a bunch of file in a directory.
your help is appreciated

Reply

19 nixCraft December 17, 2009 at 6:29 am

@gurpur

Again, offtopic, go to our forum at nixcraft.com to post all your shell scripting related queries.

Reply

20 Xa December 19, 2010 at 4:14 am

for i in ‘ls’
did NOT work for me. What does work is this:
for i in $(ls)

Reply

21 mwcz April 5, 2013 at 5:56 pm

That’s because you used single quotes instead of backticks.

It’s not this:

for i in ‘ls’

It’s this:

for i in `ls`

Reply

22 she March 7, 2011 at 4:21 pm

can somebody help me develop a program that has the option whether to overwrite a document or not. when making a file, it will ask the user if he would want to create a new file on the same file name or overwrite the said filename.. please i need this badly.

Reply

23 Benjamin June 24, 2011 at 8:12 pm

Nice compilation, thanks! One suggestion, though: I think many people would be looking for still another thing: how to get file extensions and how to get filenames without extensions.

Reply

24 TheBonsai June 25, 2011 at 2:46 pm
25 nixCraft June 25, 2011 at 3:29 pm
26 monr4 July 15, 2011 at 9:09 pm

hello, a have a problem, i need to do a script por a rutine something like

for i in $(cat list1) but with 2 list
for i in $(cat list1) AND b in $(cat list2);do mknod /dev/$i/group c64[$b];done

somebody helpme with that please

Reply

27 Hari December 7, 2011 at 12:21 pm

Hello I have a problem .i want to run a program N times by give N number of different .txt files from a directory. i want to give all the files in single time by using loops can any one help me to script .

Reply

28 Saad January 10, 2012 at 6:48 am

Is it possible to know how many files will this loop run on,

e.g. in the following loop

for f in *.pdf
do

done;

can i know how many files are present in *.pdf glob, and can i use that to know how many files have been processed out of the total number

Reply

29 RDuke January 10, 2012 at 3:32 pm

@saad: maybe something like

count=$( ls -1 *.pdf | wc -l )

Reply

30 nixCraft January 11, 2012 at 8:27 am

Pure bash code and nothing else :)

 filecount=(*.pdf)
 echo "Total files in $PWD: ${#filecount[@]}"

Reply

31 Potato January 11, 2012 at 12:26 am

try COUNT=`ls -1 *.pdf | wc -l`

Reply

32 ziming May 12, 2012 at 10:41 am

Very good tutorials. Helps me lot. Thx mate.

Reply

33 Manuel Rodriguez May 30, 2012 at 4:42 pm

good stuff

Reply

34 Mike February 14, 2013 at 7:53 am

thanks this was helpfull (still!!) ;) and I have been scripting for 20 years!

Reply

35 Inukaze October 11, 2013 at 7:51 pm

Hi there im trying to say to Bash , wait for a Process Start / Begin

im trying by this way for example

notepad=`pidof notepad.exe`
until [ $notepad > 0 ]
do
    taskset -p 03 $notepad
    renince -n 5 -p $notepad
done

well i have 2 questions:
First -> why this generate a file called “0” (the file are empty)
i dont wanna make a new file , just wait for the PID to check execution.

Second -> This is a Loop , but if the 2 commands are execute correclty , 1 time , how i can continue to done ???

Reply

36 Aaron Burns April 1, 2014 at 9:12 pm

I keep seeing “for f in “…. what is the f for. I don’t see it defined anywhere.

Reply

37 Nix Craft April 2, 2014 at 7:51 am

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