Scripting

Commands (to be interpreted and executed) can be placed in a text file, called script, to be executed by means of an interpreter

The interpreter is specified in the first line of the script, e.g. by:

 #! /bin/sh
 #! /bin/bash
 #! /bin/tcsh
 #! /usr/bin/awk -f
 #! /usr/bin/env python
 ...

(Note that while # is in all the above languages a comment, #! is actually used to identify the interpreter).

Bash scripting

Among the many, bash scripting is particularly relevant to us (bash is also the interpreter of the command-line shell we have been using so far).

Unix commands (enriched by bash built-in functions & structures) can be used in bash scripts:

$> cat ./get_users.sh

#! /bin/bash  -x
filein=/etc/passwd
#
# extract user names
cat $filein | awk -v FS=":" '{print $1}'

Note that in order to execute get_users.sh, we need to change its permissions,

 $> chmod a+x ./get_users.sh

When executing, the output fo the script can also be redirected to a file,

 $> ./get_users.sh > users.dat

Within the script, $0 corresponds to the invocation name (./get_users.sh, in the example above), $1, $2, .. $n to the n-th arguments if present. $# is the number of command line arguments passed to the script.

$> cat ./get_users2.sh

#! /bin/bash
if [ $# == 0 ] ; then echo "Usage:  ./get_users2.sh  <filename>" ; exit 1 ; fi
filein=$1
# 
# extract user names
cat $filein | awk -v FS=":" '{print $1}'

Now, this second version of the script needs to be run as:

$> ./get_users2.sh /etc/passwd