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

Sed & Awk

These two commands, available almost everywhere, are extremely used in bash scripting.

sed

substitutes regular expressions in files or strings. Examples follow:

$> echo “Ciao Ciao” | sed ‘s/C/M/’
     ->  “Miao Ciao”
$> echo “Ciao Ciao” | sed ‘s/C/M/g’
     ->  “Miao Miao”                  # g stands for “global substitution”

Regular expressions can also be used in the search.

• "." in the regular expr means all characters (wild card) and needs to be protected as \. to be treated as a regular character
• \n means newline
• \t tab

awk

line by line operations (number & strings, syntax similar to c)

$> echo 10 4.0 | awk '{print $1 * sqrt($2)}'
$> echo “LabQSM 2020” | awk '{print $1; print "Year", $2}'

Awk has its own scripting, useful eg for parsing or data post-processing (the same operation/search is done line by line)

Take eg the file apt.txt with the list of tennis players that we have used in previous examples:

 9850, Nadal,  Rafael 
 6630, Federer,  Roger
 3075, Berrettini,  Matteo 
12030, Djokovic,  Novak

The problem can be solved by awk as follows:

#! /usr/bin/awk -f
BEGIN{ i=ind; nlines=0; FS="," }
{
  if (NF != 3) next
  nlines++
  if (nlines == i) {printf "%s, %s\n", $3,$2}
}
END{
# place here any operation to be done at the end
}

Run as:

$> ./solution.awk -v ind=2  apt.txt

Note that comma-separated columns are no longer needed, and one can avoid using commas by simply dropping the redefinition of the field-separator FS=",".