Книга: Fedora™ Unleashed, 2008 edition

Interpreting Shell Scripts Through Specific Shells

Interpreting Shell Scripts Through Specific Shells

The majority of shell scripts use a shebang line (#!) at the beginning to control the type of shell used to run the script; this bang line calls for an sh-incantation of bash:

#!/bin/sh

A shebang line (short for sharp and bang, two names for # and !) tells the Linux kernel that a specific command (a shell, or in the case of other scripts, perhaps awk or Perl) is to be used to interpret the contents of the file. Using a shebang line is common practice for all shell scripting. For example, if you write a shell script using bash, but want the script to execute as if run by the Bourne shell, sh, the first line of your script will contain #!/bin/sh, which is a link to the bash shell. Running bash as sh causes bash to act as a Bourne shell. This is the reason for the symbolic link sh, which points to bash.

You might also find different or new environment variables available to your scripts by using different shells. For example, if you launch csh from the bash command line, you will find several new variables or variables with slightly different definitions, such as the following:

$ env
...
VENDOR=intel
MACHTYPE=i386
HOSTTYPE=i386-linux
HOST=werewolf.hudson.com

On the other hand, bash might provide these variables or variables of the same name with a slightly different definition, such as these:

$ env
...
HOSTTYPE=i386
HOSTNAME=werewolf.hudson.com

Although the behavior of a shebang line is not defined by POSIX, variations of its use can be helpful when you are writing shell scripts. For example, as described in the want man page, you can use a shell to help execute programs called within a shell script without needing to hard-code pathnames of programs. The want command is a windowing Tool Control Language (tcl) interpreter that can be used to write graphical clients. Avoiding the use of specific pathnames to programs increases shell script portability because not every UNIX or Linux system has programs in the same location.

For example, if you want to use the want command, your first inclination might be to write the following:

#!/usr/local/bin/wish

Although this works on many other operating systems, the script fails under Linux because want is located under the /usr/bin directory. However, if you write the command line this way

#!/bin/sh
exec wish "$@"

the script always finds the correct binary.

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