| Tutorial details | |
|---|---|
| Difficulty level | Easy |
| Root privileges | No |
| Requirements | None |
| Est. reading time | 2 minutes |
foreach n ( 1 2 3 4 5 ) #command1 #command2 end
However, bash lacks foreach syntax; instead, you can use bash while loop or bash for loop syntax as described below.
Bash foreach loop examples
Let us say you want to convert the following csh forloop example:
foreach i ( * ) echo "Working $i filename ..." end
Above csh or tcsh foreach displayed a list of files using loop. Here is similar code in a bash shell:
for i in * do echo "Working on $i file..." done
However, one can find and safely handle file names containing newlines, spaces, special characters using the find command
## tested on GNU/Linux find ## ## and bsd/find only ## find /dir/ -print0 | xargs -r0 command find /tmp/test -print0 | xargs -I {} -r0 echo "Working on "{}" file ..." find /tmp/test -type f -print0 | xargs -I {} -r0 echo "Working on '{}' file ..."
Working on '/tmp/test' file ... Working on '/tmp/test/hosts.deny' file ... Working on '/tmp/test/My Resume.pdf' file ... Working on '/tmp/test/hostname' file ... Working on '/tmp/test/hosts.allow' file ... Working on '/tmp/test/Another file name.txt' file ... Working on '/tmp/test/resolv.conf' file ... Working on '/tmp/test/hosts' file ... Working on '/tmp/test/host.conf' file ...
Howto use foreach in bash shell
Say you have a file named lists.txt as follows:
cat lists.txt
Sample outputs:
/nfs/db1.dat /nfs/db2.dat /nfs/share/sales.db /nfs/share/acct.db /nfs/private/users.db /nfs/private/payroll.db
Each in there is a file located on your Unix or Linux server. Here is how to read lists.txt file and work on each file listed in lists.txt:
for n in $(cat lists.txt ) do echo "Working on $n file name now" # do something on $n below, say count line numbers # wc -l "$n" done
Although for loop seems easy to use for reading the file, it has some problems. Don’t try to use “for” to read file line by line in Linux or Unix. Instead, use while loop as follows:
#!/bin/bash ## path to input file input="lists.txt" ## Let us read a file line-by-line using while loop ## while IFS= read -r line do printf 'Working on %s file...\n' "$line" done < "$input"
🐧 3 comments so far... add one ↓
| Category | List of Unix and Linux commands |
|---|---|
| Disk space analyzers | df • ncdu • pydf |
| File Management | cat • cp • mkdir • tree |
| Firewall | Alpine Awall • CentOS 8 • OpenSUSE • RHEL 8 • Ubuntu 16.04 • Ubuntu 18.04 • Ubuntu 20.04 |
| Network Utilities | NetHogs • dig • host • ip • nmap |
| OpenVPN | CentOS 7 • CentOS 8 • Debian 10 • Debian 8/9 • Ubuntu 18.04 • Ubuntu 20.04 |
| Package Manager | apk • apt |
| Processes Management | bg • chroot • cron • disown • fg • jobs • killall • kill • pidof • pstree • pwdx • time |
| Searching | grep • whereis • which |
| User Information | groups • id • lastcomm • last • lid/libuser-lid • logname • members • users • whoami • who • w |
| WireGuard VPN | Alpine • CentOS 8 • Debian 10 • Firewall • Ubuntu 20.04 |
How would one do that example command on a single line command in MacOS’ terminal? Tried remove the line breaks and tabs and then it becomes non-functional. Tried adding some \newline s even and it still didn’t work.
My issue when going from tcsh to bash is that if you use a command in the loop that is capable of *optionally* taking STDIN *if* it is present, the loop stops if you are using the contents of the file as the loop items. tcsh does not have this problem.
Let’s say we have a command called cat.pl that can operate on a given file name, but if STDIN is present, will take STDIN instead and ignore the supplied files. In that instance, this tcsh loop has no problem what-so-ever:
However, if you want to do that in bash, using a while loop as suggested, there’s a loose canon of an input stream available to the commands inside the loop to consume, so:
Then cat.pl output all except the first line of filelist.txt and the loop only executes 1 iteration.
This caught me by surprise when I first started learning bash, trying to do what I normally would do in tcsh. And for the life of me, I cannot figure out 1 use-case why you would supply the loop items of the standard input to the commands inside the loop.
Personally, to me, having used tcsh for so long, it seems sloppy to allow the stream of input to the loop controls to be available to the commands inside the loop.
Sure, bash has a lot of extras that make it better, but this one issue has always bothered me. I would be interested in hearing how you deal with this situation. All of the scripts I have written over the years while being a tcsh user optionally take STDIN if it is present and/or takes explicitly supplied files.
Instead of:
Try:
while IFS= read -r line do echo "$line" done < "${1:-/dev/stdin}"Now we can run it as to read list from filelist.txt
This will read from STDIN
I hope this helps!