Word Splitting


The shell's parser performs several operations on your commands before finally executing them. Understanding how your original command will be transformed by the shell is of paramount importance in writing robust scripts. From the bash man page:

For additional information on word splitting and argument handling in Bash, consider reading Arguments.

What is Word Splitting?

This page will focus on word splitting, of course. Before we get into the technical details, let's write a little helper script that will show us the arguments as passed by the shell:

#!/bin/sh -
printf "%d args:" "$#"
[ "$#" -eq 0 ] || printf " <%s>" "$@"

If you create a file named args with the above contents, make it executable with chmod a+x args , and put it in one of the directories listed in echo "$PATH" , then you could run the following command with the following output:

griffon:~$ args hello world "how are you?"
3 args: <hello> <world> <how are you?>

Our helper program above receives the argument list as constructed by the shell, and shows it to us.

The ultimate result of most shell commands is to execute some program with a specific set of arguments (as well as setting up environment variables, opening file descriptors, etc.).

The above command is code in the shell language where space is used to delimit words/tokens, and quotes are used so remove the special role of some characters (in this instance of space as word delimiter and ? as a glob operator). That line split into word is not what we call Word Splitting, it is just shell code syntax parsing.

Word splitting is a separate process that happens after syntax parsing into tokens and before pathname expansion (aka filename generation or globbing) and that is performed on the results of almost all unquoted expansions. The result of the expansion is broken into separate words based on the characters of the IFS variable. If IFS is not set, then it will be performed as if IFS contained a space, a tab, and a newline. For example:

griffon:~$ var="This is a variable"
griffon:~$ args $var
4 args: <This> <is> <a> <variable>

An example combining word splitting and pathname expansion performed both upon parameter expansion:

griffon:~$ var="Some wildcard /b* characters"
griffon:~$ args $var
5 args: <Some> <wildcard> </bin> </boot> <characters>

An example using IFS:

griffon:~$ log=/var/log/qmail/current IFS=/
griffon:~$ args $log
5 args: <> <var> <log> <qmail> <current>
griffon:~$ unset IFS

An example with CommandSubstitution:

griffon:/music/Yello$ ls -l
total 2864
-rw-r--r-- 1 greg greg 2919154 2001-05-23 00:48 Yello - Oh Yeah.mp3

griffon:/music/Yello$ args $(ls -l)
11 args: <-rw-r--r--> <1> <greg> <greg> <2919154> <2001-05-23> <00:48> <Yello> <-> <Oh> <Yeah.mp3>

An example with ArithmeticExpansion:

griffon:~$ IFS=2
griffon:~$ args $(( 11 * 11 ))
2 args: <1> <1>

Controlling Word Splitting

As you can see above, we usually do not want to let word splitting nor pathname expansion occur upon expansions when filenames are involved. (See BashPitfalls for a discussion of this particular issue.)

Double quoting an expansion suppresses word splitting, except in the special cases of "$@" and "${array[@]}" as well as pathname expansion:

griffon:~$ var="This is a variable"; args "$var"
1 args: <This is a variable>

griffon:~$ array=(testing, testing, "1 2 3"); args "${array[@]}"
3 args: <testing,> <testing,> <1 2 3>

"$@" causes each positional parameter to be expanded to a separate word; its array equivalent likewise causes each element of the array to be expanded to a separate word.

There are very complicated rules involving whitespace characters in IFS. Quoting the man page again:

We won't explore those rules in depth here, except to note the part about sequences of non-whitespace characters. If IFS contains non-whitespace characters, then empty words can be generated:

griffon:~$ getent passwd sshd

griffon:~$ IFS=:; args $(getent passwd sshd)
7 args: <sshd> <x> <100> <65534> <> </var/run/sshd> </usr/sbin/nologin>
griffon:~$ unset IFS

There was another empty word generated in one of our previous examples, where IFS was set to :. The observant reader will have noticed, therefore, that non-whitespace IFS characters are not ignored at the beginning and end of expansions, the way whitespace IFS characters are.

Whitespace IFS characters get consolidated. Multiple spaces in a row, for example, have the same effect as a single space, when IFS contains a space (or is not set at all). Newlines also count as whitespace for this purpose, which has important ramifications when attempting to load an array with lines of input.

Finally, we note that pathname expansion happens after word splitting, and can produce some very shocking results.

griffon:~$ getent passwd qmaild

griffon:~$ IFS=:; args $(getent passwd qmaild)
737 args: <qmaild> <00INDEX.lsof> <03> <037_ftpd.patch> ...
griffon:~$ unset IFS

The * word, produced by the shell's word splitting, was then expanded as a glob, resulting in several hundred new and exciting words. This can be disastrous if it happens unexpectedly. As with most of the dangerous features of the shell, it is retained because "it's always worked that way". In fact, it could be used for good, if you're very careful:

griffon:/music/Yello$ files='*.mp3 *.ogg'
griffon:/music/Yello$ args $files
2 args: <Yello - Oh Yeah.mp3> <*.ogg>

Though with a shell with array support such as Bash, you'd generally want $files being an array containing the files rather than a space-separated list of patterns:

griffon:/music/Yello$ files=(*.mp3 *.ogg)
griffon:/music/Yello$ args "${files[@]}"
2 args: <Yello - Oh Yeah.mp3> <*.ogg>

If word splitting is desired, then you generally also need to disable pathname expansion with set -o noglob or set -f.

With bash 4.4 or newer, option settings can be made local to a function which allows the noglob option in addition to the $IFS parameter to be set temporarily for the purpose of doing one splitting operation without affecting the overall behaviour of the shell with local - (copied from the Almquist shell where the idea is to make $- local):

getuser() {
  local - IFS=: user="$1"
  set -o noglob
  user_fields=( $(getent -- passwd "$user") )



WordSplitting (last edited 2023-09-21 06:22:37 by StephaneChazelas)