11.3 Special Variables
Most shell scripts understand command-line parameters and interact with the commands that they run. To take your scripts from being just a simple list of commands to becoming more flexible shell script programs, you need to know how to use the special Bourne shell variables. These special variables are like any other shell variable as described in Section 2.8, except that you can’t change the values of certain ones.
After reading the next few sections, you’ll understand why shell scripts accumulate many special characters as they are written. If you’re trying to understand a shell script and you come across a line that looks completely incomprehensible, pick it apart piece by piece.
11.3.1 Individual Arguments: $1, $2, and So On
$1, $2, and all variables named as positive nonzero integers contain the values of the script parameters, or arguments. For example, say the name of the following script is pshow:
#!/bin/sh
echo First argument: $1
echo Third argument: $3
Try running the script as follows to see how it prints the arguments:
$ ./pshow one two three
First argument: one
Third argument: three
The built-in shell command shift can be used with argument variables to remove the first argument ($1) and advance the rest of the arguments so that $2 becomes $1, $3 becomes $2, and so on. For example, assume that the name of the following script is shiftex:
#!/bin/sh
echo Argument: $1
shift
echo Argument: $1
shift
echo Argument: $1
Run it like this to see it work:
$ ./shiftex one two three
Argument: one
Argument: two
Argument: three
As you can see, shiftex prints all three arguments by printing the first, shifting the remaining arguments, and repeating.
11.3.2 Number of Arguments: $#
The $# variable holds the number of arguments passed to a script and is especially important when you’re running shift in a loop to pick through arguments. When $# is 0, no arguments remain, so $1 is empty. (See Section 11.6 for a description of loops.)
11.3.3 All Arguments: $@
The $@ variable represents all of a script’s arguments and is very useful for passing them to a command inside the script. For example, Ghostscript commands (gs) are usually long and complicated. Suppose you want a shortcut for rasterizing a PostScript file at 150 dpi, using the standard output stream, while also leaving the door open for passing other options to gs. You could write a script like this to allow for additional command-line options:
#!/bin/sh
gs -q -dBATCH -dNOPAUSE -dSAFER -sOutputFile=- -sDEVICE=pnmraw $@
If a line in your shell script gets too long , making it difficult to read and manipulate in your text editor, you can split it up with a backslash ( \). For example, you can alter the preceding script as follows:
#!/bin/sh
gs -q -dBATCH -dNOPAUSE -dSAFER \
-sOutputFile=- -sDEVICE=pnmraw $@
11.3.4 Script Name: $0
The $0 variable holds the name of the script and is useful for generating diagnostic messages. For example, say your script needs to report an invalid argument that is stored in the $BADPARM variable. You can print the diagnostic message with the following line so that the script name appears in the error message:
echo $0: bad option $BADPARM
All diagnostic error messages should go to the standard error. As explained in Section 2.14.1, 2>&1 redirects the standard error to the standard output. For writing to the standard error, you can reverse the process with 1>&2. To do this for the preceding example, use this:
echo $0: bad option $BADPARM 1>&2
11.3.5 Process ID: $$
The $$ variable holds the process ID of the shell.
11.3.6 Exit Code: $?
The $? variable holds the exit code of the last command that the shell executed. Exit codes, which are critical to mastering shell scripts, are discussed next.