Bash Shell Loop Over Set of Files

Posted on in Categories , , , , , , , , , , , , last updated June 29, 2010

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!

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

Share this on:

47 comment

    1. What if you have 5 or 6 files, specific files .biz in a directory and you want to store them one at the time in a variable $mirage, and want to process $mirage until no more .biz file are found.
      How will you modify the script accordingly

      Reply Report comment

  3. 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 Report comment

    1. What if you have 5 or 6 files, specific files .biz in a directory and you want to store them one at the time in a variable $mirage, and want to process $mirage until no more .biz file are found.
      How will you modify the script accordingly

      Reply Report comment

  6. 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 Report comment

  7. 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 Report comment

  11. 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 Report comment

  12. > 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 Report comment

  13. 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 Report comment

  14. 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 Report comment

  15. 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 Report comment

  16. 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 Report comment

  17. @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 Report comment

  20. 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 Report comment

  21. 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 Report comment

  22. 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 Report comment

  23. 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 Report comment

  24. 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 Report comment

  29. 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 Report comment

  31. Hi All,
    I want to write a shell script for processing the input files(file.csv) from one path and redirect to the other mount or same mount for required query based on the acknowledgment file name (same as input file but file.done) .Please help me

    Reply Report comment

  32. It should be noted that none of the above examples work if you want to explicitly specify a directory to iterate through (instead of the current directory, aka *) that has a space in it. I still can’t figure out how to do that. If I set a variable to the explicit path, neither quotes nor delimiters allow iteration through that directory; the script’s attempt to navigate to that directory stops at the space as if a new argument started after it. And since this isn’t that unusual a situation, it’s unfortunate nobody covered it above. Everyone got stuck on using the current directory, which I would think is actually a less-versatile construct.

    Reply Report comment

  34. It seems to be a nice thread.

    I’m having one similar issue and I guess Unix Loop might help.

    I need a write a cleanup script in unix shell to delete files and folders older than 3 days but the problem is that I need to exclude .snp files/folders. And that can be present any where in the directory. I tried with normal “find” command but that is not excluding the file from nested directory…
    It seems only loop can help…

    Any idea?

    Reply Report comment

    1. Continue to use your find command. You can match the inverse of patterns with grep. The following will return everything-but what is specified in the pattern.

      grep -v somepatternIwanttoexclude

      so for example,
      find blah blah | grep -v .snp | grep -v .snpFolder

      Reply Report comment

  37. It is worth noting that none of these examples that use * will scale. It is much more reliable to use the find command. To “loop” over every file in /dir and run some_command :

    find /dir -exec some_command {} \;

    HINT: find replaces {} with the current filename and \; marks the end of the command.

    Reply Report comment

Leave a Comment

Tagged as: Tags , , , , , , , , , , , , , , , , , , , , , ,