Databases, Systems & Networks

Bash Shell Loop

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

Source: nixCraft