Diff for "BashFAQ/073"

How can I use parameter expansion? How can I get substrings? How can I get a file without its extension, or get just a file's extension?

Your shell has variables (either inherited from the environment, or defined by you), as well as several other special parameters like $ (the shell's process ID) and so on. All of these are considered parameters, and they can be expanded by putting a $ in front. However, that's just the simplest case....

Parameter Expansion is far more general and powerful than that. (The full set of capabilities can be found in the bash manpage, or see the reference or the bash hackers article about it). It can be hard to understand parameter expansion without actually using it, so we're going to begin with some examples.

The first set of capabilities involves removing a substring, from either the beginning or the end of a parameter. Here's an example using parameter expansion with something akin to a hostname (dot-separated components):

parameter     result
-----------   ------------------------------
${NAME}       polish.ostrich.racing.champion
${NAME#*.}           ostrich.racing.champion
${NAME##*.}                         champion
${NAME%%.*}   polish                        
${NAME%.*}    polish.ostrich.racing         

And here's an example of the parameter expansions for a typical filename:

parameter     result
-----------   --------------------------------------------------------
${FILE}       /usr/share/java-1.4.2-sun/demo/applets/Clock/Clock.class
${FILE#*/}     usr/share/java-1.4.2-sun/demo/applets/Clock/Clock.class
${FILE##*/}                                                Clock.class
${FILE%%/*}                                                           
${FILE%/*}    /usr/share/java-1.4.2-sun/demo/applets/Clock            

US keyboard users may find it helpful to observe that, on the keyboard, the "#" is to the left of the "%" symbol. Mnemonically, "#" operates on the left side of a parameter, and "%" operates on the right. The glob after the "%" or "%%" or "#" or "##" specifies what pattern to remove from the parameter expansion.

You cannot nest parameter expansions. If you need to perform two expansions steps, use a variable to hold the result of the first expansion:

# foo holds: key="some value"
bar=${foo#*=\"} bar=${bar%\"*}
# now bar holds: some value

Here are a few more examples (but please see the real documentation for a list of all the features!). I include these mostly so people won't break the wiki again, trying to add new questions that answer this stuff.

${string:2:1}   # The third character of string (0, 1, 2 = third)
${string:1}     # The string starting from the second character
                # Note: this is equivalent to ${string#?}
${string%?}     # The string with its last character removed.
${string: -1}   # The last character of string
${string:(-1)}  # The last character of string, alternate syntax
                # Note: string:-1 means something entirely different; see below.

${file%.mp3}    # The filename without the .mp3 extension
                # Very useful in loops of the form: for file in *.mp3; do ...
${file%.*}      # The filename without its extension (assuming there was
                # only one extension in the first place...).
${file%%.*}     # The filename without all of its extensions
${file##*.}     # The extension only.

Bash 4

Bash 4 introduces some additional parameter expansions; toupper (^) and tolower (,).

# string='hello, World!'
parameter     result
-----------   --------------------------------------------------------
${string^}    Hello, World! # First character to uppercase
${string^^}   HELLO, WORLD! # All characters to uppercase
${string,}    hello, World! # First character to lowercase
${string,,}   hello, world! # All characters to lowercase

Parameter Expansion on Arrays

BASH arrays are remarkably flexible, since they are so well integrated with the other shell expansions. Virtually any expansion you can carry out on a scalar can equally be applied to a whole array. Remember that quoting an array expansion using @ (e.g. "$@" or "${cmd[@]}") results in the members being treated as individual words, regardless of their content. So for example, arr=("${list[@]}" foo) correctly handles all elements in the list array.

First the expansions:

$ a=(alpha beta gamma)  # our base array
$ echo "${a[@]#a}"      # chop 'a' from the beginning of every member
lpha beta gamma
$ echo "${a[@]%a}"      # from the end
alph bet gamm
$ echo "${a[@]//a/f}"   # substitution
flphf betf gfmmf

The following expansions (substitute at beginning or end) are very useful for the next part:

$ echo "${a[@]/#a/f}"   # substitute a for f at start
flpha beta gamma
$ echo "${a[@]/%a/f}"   # at end
alphf betf gammf

We use these to prefix or suffix every member of the list:

$ echo "${a[@]/#/a}"    # append a to beginning
aalpha abeta agamma     #    (thanks to floyd-n-milan for this)
$ echo "${a[@]/%/a}"    # append a to end
alphaa betaa gammaa

This works by substituting an empty string at beginning or end with the value we wish to append.

So finally, a quick example of how you might use this in a script, say to add a user-defined prefix to every target:

$ PFX=inc_
$ a=("${a[@]/#/$PFX}")
$ echo "${a[@]}"
inc_alpha inc_beta inc_gamma

This is very useful, as you might imagine, since it saves looping over every member of the array.

The special parameter @ can also be used as an array for purposes of parameter expansions:

${@:(-2):1}             # the second-to-last parameter
${@: -2:1}              # alternative syntax

You can't use ${@:-2:1}, however, because that collides with the syntax in the next section.

Portability

The original Bourne shell (7th edition Unix) only supports a very limited set of parameter expansion options:

${var-word}             # if var is defined, use var; otherwise, "word"
${var+word}             # if var is defined, use "word"; otherwise, nothing
${var=word}             # if var is defined, use var; otherwise, use "word" AND...
                        #   also assign "word" to var
${var?error}            # if var is defined, use var; otherwise print "error" and exit

These are the only completely portable expansions available.

POSIX shells (as well as KornShell and BASH) offer those, plus a slight variant:

${var:-word}             # if var is defined AND NOT EMPTY, use var; otherwise, "word"
etc.

POSIX, Korn (all versions) and Bash all support the ${var#word}, ${var%word}, ${var##word} and ${var%%word} expansions.

ksh88 does not support ${var/replace/with} or ${var//replace/all}, but ksh93 and Bash do.

ksh88 does not support fancy expansion with arrays (e.g., ${a[@]%.gif}) but ksh93 and Bash do.