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? What are some good ways to do basename and dirname?

Parameter expansion is an important subject. This page contains a concise overview of parameter expansion.

Parameter Expansion substitutes a variable or special parameter for its value. It is the primary way of dereferencing (referring to) variables in Bourne-like shells such as Bash. Parameter expansion can also perform various operations on the value at the same time for convenience.

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. Another mnemonic is that in an English sentence "#" usually comes before a number (e.g., "The #1 Bash reference site"), while "%" usually comes after a number (e.g., "Now 5% discounted"), so they operate on those sides.

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 last extension
${file%%.*}     # The filename without all of its extensions
${file##*.}     # The extension only, assuming there is one. If not, will expand to ${file}

Examples of Filename Manipulation

Here is one Posix-compliant way to take a full pathname, extract the directory component of the pathname, the filename, just the extension, the filename without the extension (the "stub"), any numeric portion occurring at the end of the stub (ignoring any digits that occur in the middle of the filename), perform arithmetic on that number (in this case, incrementing by one), and reassemble the entire filename adding a prefix to the filename and replacing the number in the filename with another one.

FullPath=/path/to/name4afile-00809.ext   # result:   #   /path/to/name4afile-00809.ext
Filename=${FullPath##*/}                             #   name4afile-00809.ext
PathPref=${FullPath%"$Filename"}                     #   /path/to/
FileStub=${Filename%.*}                              #   name4afile-00809
FileExt=${Filename#"$FileStub"}                      #   .ext
FnumPossLeading0s=${FileStub##*[![:digit:]]}         #   00809
FnumOnlyLeading0s=${FnumPossLeading0s%%[!0]*}        #   00
FileNumber=${FnumPossLeading0s#"$FnumOnlyLeading0s"} #   809
NextNumber=$(( FileNumber + 1 ))                     #   810
FileStubNoNum=${FileStub%"$FnumPossLeading0s"}       #   name4afile-
NewFullPath=${PathPref}New_${FileStubNoNum}${FnumOnlyLeading0s}${NextNumber}${FileExt}
                        # Final result is:           #   /path/to/New_name4afile-00810.ext

Note that trying to get the directory component of the pathname with PathPref="${FullPath%/*}" will fail to return an empty string if $FullPath is "SomeFilename.ext" or some other pathname without a slash. Similarly, trying to get the file extension using FileExt="${Filename#*.}" fails to return an empty string if $Filename has no dot (and thus no extension).

Also note that it is necessary to get rid of leading zeroes for $FileNumber in order to perform arithmetic, or else the number is interpreted as octal. Alternatively, one can add a 10# prefix to force base 10. In the example above, trying to calculate $(( FnumPossLeading0s + 1 )) results in an error since "00809" is not a valid number. If we had used "00777" instead, then there would have been no error, but $(( FnumPossLeading0s + 1 )) would result in "1000" (since octal 777 + 1 is octal 1000) which is probably not what was intended. See ArithmeticExpression.

Quoting is not needed in variable assignment, since WordSplitting does not occur. On the other hand, variables referenced inside a parameter expansion need to be quoted (for example, quote $Filename in PathPref=${FullPath%"$Filename"} ) or else any * or ? or other such characters within the filename would incorrectly become part of the parameter expansion (for example, if an asterisk is the first character in the filename --try FullPath="dir/*filename" ).

The example above fails to compensate if the result of the arithmetic operation, $NextNumber, has a different number of digits than the original. If $FullPath were "filename099" then $NewFullPath would have been "New_filename0100" with the filename being one digit longer.

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, because they are well integrated with the other shell expansions. Any parameter expansion that can be carried out on a scalar or individual array element can equally apply to an entire array or the set of positional parameters such that all members are expanded at once, possibly with an additional operation mapped across each element. This is done by expanding parameters of the form @, *, arrayname[@] and arrayname[*]. It is critical that these special expansions be Quoted properly - almost always that means double-quoting (e.g. "$@" or "${cmd[@]}") - so that the members are treated literally as individual words, regardless of their content. For example, arr=("${list[@]}" foo) correctly handles all elements in the list array.

First the expansions:

$ a=(alpha beta gamma)  # assign to our base array via compound assignment
$ 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} (note the whitespace), however, because that conflicts with the syntax described 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"
similarly for ${var:+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.

ksh88 does not support the arr=(...) style of compound assignment. Either use set -A arrayname -- elem1 elem2 ..., or assign each element individually with arr[0]=val1 arr[1]=val2 ...

BashFAQ/073 (last edited 2013-12-08 13:27:30 by GreyCat)