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 "${lb2file}" done |
Updated for accuracy!
The scripts are wrong, use `$FILES` (without quotes) in the loop instead of `”$FILES”`.
DAY,
I don’t think so it is wrong. Do you have any problem running script?
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
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
Hmm…not sure why you wouldn’t use
for file in *
or
for file in *.c
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 $mirage until no more .biz file are found.
How will you modify the script accordingly
DAY,
Hmm, it should work (I hope you have *.c files in current directory) or try as suggested by brock.
Thanks, vivek and Brock. Replacing “$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. $f store current file name
done
OK, read sth. in the bash man. page.
Quoting is used to remove the special meaning of certain characters or words to the shell. Quoting can be used to disable special treatment for special characters, to prevent reserved words from being recognized as such, and to prevent parameter expansion.
I think that’s why the original script doesn’t work. “$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 `*’.
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.
Forgot to share a little bashism that makes it easy to determine a good guess of the ARG_MAX:
MAXARGS=$(( $(getconf ARG_MAX) – $(env | wc -c) ))
I use loops like this a lot,
for x in * ; do
test -f “$x” || continue
COMMAND “$x”
done
helps to easily ignore subdirectories
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)
> FILES=â€*†is wrong unless you want the value of $FILES to be *. The same is true of “*.câ€. 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!
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
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="*"
andFILES=*
are equivalent since sh-compliant shells don’t expand globs during variable assignment.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 '$file' " ; done
the file variable is now ‘*.jpg’
I would have expected that the contents of the for loop would not be executed at all, since there was no jpg file.
Found the problem with my previous example. The nullglob shell option needs to be set:
shopt -s nullglob; for file in *.jpg; do echo " the file variable is now '$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'
@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.
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
@gurpur
Again, offtopic, go to our forum at nixcraft.com to post all your shell scripting related queries.
for i in ‘ls’
did NOT work for me. What does work is this:
for i in $(ls)
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`
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.
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.
http://wiki.bash-hackers.org/syntax/pe#common_use
Also, see HowTo: Use Bash Parameter Substitution Like A Pro
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[$b];done
somebody helpme with that please
Hello I have a problem .i want to run a program N times by give N number of different .txt files from a directory. i want to give all the files in single time by using loops can any one help me to script .
Is it possible to know how many files will this loop run on,
e.g. in the following loop
for f in *.pdf
do
done;
can i know how many files are present in *.pdf glob, and can i use that to know how many files have been processed out of the total number
@saad: maybe something like
count=$( ls -1 *.pdf | wc -l )
Pure bash code and nothing else :)
try COUNT=`ls -1 *.pdf | wc -l`
Very good tutorials. Helps me lot. Thx mate.
good stuff
thanks this was helpfull (still!!) ;) and I have been scripting for 20 years!
Hi there im trying to say to Bash , wait for a Process Start / Begin
im trying by this way for example
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 ???
I keep seeing “for f in “…. what is the f for. I don’t see it defined anywhere.
f is a shell variable see – https://www.cyberciti.biz/faq/bash-for-loop/ or http://bash.cyberciti.biz/guide/Main_Page
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
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.
Very good tutorial, Thanks.
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?
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
Very usefull!
Thank you!
att,
shopt -s nullglob helped me. Thanks
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.