Shell Script Examples
This section presents several shell script examples.
Using Arguments
Example 10. Shell Script Arguments
#!/bin/bash # example of using arguments to a script echo "My first name is $1" echo "My surname is $2" echo "Total number of arguments is $#" |
Save this file as name.sh, set execute permission on that file by typing chmod a+x name.sh and then execute the file like this: ./name.sh.
$ chmod a+x name.sh $ ./name.sh Hans-Wolfgang Loidl My first name is Hans-Wolfgang My surname is Loidl Total number of arguments is 2 |
Version 1: Line count example
The first example simply counts the number of lines in an input file. It does so by iterating over all lines of a file
using a while loop, performing a read operation in the loop header.
While there is a line to process, the loop body will be executed in this case simply
increasing a counter by ((counter++)). Additionally the
current line is written into a file, whose name is specified by the variable file,
by echoing the value of the variable line and redirecting the standard output of the
variable to $file.
the current line to file. The latter is not needed for the line count, of course,
but demonstrates how to check for success of an operation: the special variable
$? will contain the return code from the previous command
(the redirected echo). By Unix convention, success is indicated
by a return code of 0, all other values are error code with application specific
meaning.
Another important issue to consider is that the integer variable, over which iteration is performed should always count down so that the analysis can find a bound. This might require some restructuring of the code, as in the following example, where an explicit counter z is introduced for this purpose. After the loop, the line count and the contents of the last line are printed, using echo. Of course, there is a Linux command that already implements line-count functionality: wc (for word-count) prints, when called with option -l, the number of lines in the file. We use this to check wether our line count is correct, demonstrating numeric operations on the way.
#!/bin/bash
# Simple line count example, using bash
#
# Bash tutorial: http://linuxconfig.org/Bash_scripting_Tutorial#8-2-read-file-into-bash-array
# My scripting link: http://www.macs.hw.ac.uk/~hwloidl/docs/index.html#scripting
#
# Usage: ./line_count.sh file
# -----------------------------------------------------------------------------
# Link filedescriptor 10 with stdin
exec 10<&0
# stdin replaced with a file supplied as a first argument
exec < $1
# remember the name of the input file
in=$1
# init
file="current_line.txt"
let count=0
# this while loop iterates over all lines of the file
while read LINE
do
# increase line counter
((count++))
# write current line to a tmp file with name $file (not needed for counting)
echo $LINE > $file
# this checks the return code of echo (not needed for writing; just for demo)
if [ $? -ne 0 ]
then echo "Error in writing to file ${file}; check its permissions!"
fi
done
echo "Number of lines: $count"
echo "The last line of the file is: `cat ${file}`"
# Note: You can achieve the same by just using the tool wc like this
echo "Expected number of lines: `wc -l $in`"
# restore stdin from filedescriptor 10
# and close filedescriptor 10
exec 0<&10 10<&-
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As documented at the start of the script, it is called like this (you must have a file text_file.txt in your current directory):
$ ./line_count.sh text_file.txt |
 | Sample text file | |
|---|---|---|
You can get a sizable sample text file by typing:
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Several versions of line counting across a set of files
This section develops several shell scripts, each counting the total number of lines across a set of files. These examples elaborate specific shell features. For counting the number of lines in one file we use wc -l. As a simple exercise you can replace this command with a call to the line counting script above.
Version 1: Explicit For loop
We use a for-loop to iterate over all files provided as arguments to the script.
We can access all arguments through the variable $*.
The sed command matches the line count, and replaces the entire line with just
the line count, using the back reference to the first substring (\1).
In the for-loop, the shell variable n is a counter for the number of files,
and s is the total line count so far.
#!/bin/bash
# Counting the number of lines in a list of files
# for loop over arguments
if [ $# -lt 1 ]
then
echo "Usage: $0 file ..."
exit 1
fi
echo "$0 counts the lines of code"
l=0
n=0
s=0
for f in $*
do
l=`wc -l $f | sed 's/^\([0-9]*\).*$/\1/'`
echo "$f: $l"
n=$[ $n + 1 ]
s=$[ $s + $l ]
done
echo "$n files in total, with $s lines in total"
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Version 2: Using a Shell Function
In this version we define a function count_lines that counts the number
of lines in the file provided as argument. Inside the function the value of the argument
is retrieved by accessing the variable $1.
#!/bin/bash
# Counting the number of lines in a list of files
# function version
count_lines () {
local f=$1
# this is the return value, i.e. non local
l=`wc -l $f | sed 's/^\([0-9]*\).*$/\1/'`
}
if [ $# -lt 1 ]
then
echo "Usage: $0 file ..."
exit 1
fi
echo "$0 counts the lines of code"
l=0
n=0
s=0
while [ "$*" != "" ]
do
count_lines $1
echo "$1: $l"
n=$[ $n + 1 ]
s=$[ $s + $l ]
shift
done
echo "$n files in total, with $s lines in total"
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Version 3: Using a return code in a function
This version tries to use the return value of the function to return the line count. However, this fails on files with more than 255 lines. The return value is intended to just provide a return code, e.g. 0 for success, 1 for failure, but not for returning proper values.
#!/bin/bash
# Counting the number of lines in a list of files
# function version using return code
# WRONG version: the return code is limited to 0-255
# so this script will run, but print wrong values for
# files with more than 255 lines
count_lines () {
local f=$1
local m
m=`wc -l $f | sed 's/^\([0-9]*\).*$/\1/'`
return $m
}
if [ $# -lt 1 ]
then
echo "Usage: $0 file ..."
exit 1
fi
echo "$0 counts the lines of code"
l=0
n=0
s=0
while [ "$*" != "" ]
do
count_lines $1
l=$?
echo "$1: $l"
n=$[ $n + 1 ]
s=$[ $s + $l ]
shift
done
echo "$n files in total, with $s lines in total"
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Version 4: Generating the file list in a shell function
#!/bin/bash
# Counting the number of lines in a list of files
# function version
# function storing list of all files in variable files
get_files () {
files="`ls *.[ch]`"
}
# function counting the number of lines in a file
count_lines () {
local f=$1 # 1st argument is filename
l=`wc -l $f | sed 's/^\([0-9]*\).*$/\1/'` # number of lines
}
# the script should be called without arguments
if [ $# -ge 1 ]
then
echo "Usage: $0 "
exit 1
fi
# split by newline
IFS=$'\012'
echo "$0 counts the lines of code"
# don't forget to initialise!
l=0
n=0
s=0
# call a function to get a list of files
get_files
# iterate over this list
for f in $files
do
# call a function to count the lines
count_lines $f
echo "$f: $l"
# increase counter
n=$[ $n + 1 ]
# increase sum of all lines
s=$[ $s + $l ]
done
echo "$n files in total, with $s lines in total"
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Version 5: Using an array to store all line counts
The example below uses shell arrays to store all filenames (file) and
its number of lines (line). The elements in an array are referred to
using the usual [ ] notation, e.g. file[1] refers to the first element in the
array file.
Note, that bash only supports 1-dimensional arrays with integers as indizes.
See the section on arrays in the Advanced Bash-Scripting Guide:.
#!/bin/bash
# Counting the number of lines in a list of files
# function version
# function storing list of all files in variable files
get_files () {
files="`ls *.[ch]`"
}
# function counting the number of lines in a file
count_lines () {
f=$1 # 1st argument is filename
l=`wc -l $f | sed 's/^\([0-9]*\).*$/\1/'` # number of lines
}
# the script should be called without arguments
if [ $# -ge 1 ]
then
echo "Usage: $0 "
exit 1
fi
# split by newline
IFS=$'\012'
echo "$0 counts the lines of code"
# don't forget to initialise!
l=0
n=0
s=0
# call a function to get a list of files
get_files
# iterate over this list
for f in $files
do
# call a function to count the lines
count_lines $f
echo "$f: $l"loc
# store filename in an array
file[$n]=$f
# store number of lines in an array
lines[$n]=$l
# increase counter
n=$[ $n + 1 ]
# increase sum of all lines
s=$[ $s + $l ]
done
echo "$n files in total, with $s lines in total"
i=5
echo "The $i-th file was ${file[$i]} with ${lines[$i]} lines"
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Version 6: Count only files we own
#!/bin/bash
# Counting the number of lines in a list of files
# for loop over arguments
# count only those files I am owner of
if [ $# -lt 1 ]
then
echo "Usage: $0 file ..."
exit 1
fi
echo "$0 counts the lines of code"
l=0
n=0
s=0
for f in $*
do
if [ -O $f ] # checks whether file owner is running the script
then
l=`wc -l $f | sed 's/^\([0-9]*\).*$/\1/'`
echo "$f: $l"
n=$[ $n + 1 ]
s=$[ $s + $l ]
else
continue
fi
done
echo "$n files in total, with $s lines in total"
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Version 7: Line count over several files
The final example supports options that can be passed from the command-line,
e.g. by ./loc7.sh -d 1 loc7.sh. The getopts
shell function is used to iterate over all options (given in the following string)
and assigning the current option to variable name.
Typically it is used in a while loop, to set shell variables that will be used later.
We use a pipe of cat and awk to print the
header of this file, up to the first empty line, if the help option is chosen.
The main part of the script is a for loop over all non-option command-line arguments.
In each iteration, $f contains the name of the file to process.
If the date options are used to narrow the scope of files to process, we use the
date and an if-statement, to compare whether the modification time
of the file is within the specified interval. Only in this case do we count the number of lines as before.
After the loop, we print the total number of lines and the number of files that have been processed.
Example 11. Version 7: Line count over several files
#!/bin/bash
############################################################################
#
# Usage: loc7.sh [options] file ...
#
# Count the number of lines in a given list of files.
# Uses a for loop over all arguments.
#
# Options:
# -h ... help message
# -d n ... consider only files modified within the last n days
# -w n ... consider only files modified within the last n weeks
#
# Limitations:
# . only one option should be given; a second one overrides
#
############################################################################
help=0
verb=0
weeks=0
# defaults
days=0
m=1
str="days"
getopts "hvd:w:" name
while [ "$name" != "?" ] ; do
case $name in
h) help=1;;
v) verb=1;;
d) days=$OPTARG
m=$OPTARG
str="days";;
w) weeks=$OPTARG
m=$OPTARG
str="weeks";;
esac
getopts "hvd:w:" name
done
if [ $help -eq 1 ]
then no_of_lines=`cat $0 | awk 'BEGIN { n = 0; } \
/^$/ { print n; \
exit; } \
{ n++; }'`
echo "`head -$no_of_lines $0`"
exit
fi
shift $[ $OPTIND - 1 ]
if [ $# -lt 1 ]
then
echo "Usage: $0 file ..."
exit 1
fi
if [ $verb -eq 1 ]
then echo "$0 counts the lines of code"
fi
l=0
n=0
s=0
for f in $*
do
x=`stat -c "%y" $f`
# modification date
d=`date --date="$x" +%y%m%d`
# date of $m days/weeks ago
e=`date --date="$m $str ago" +%y%m%d`
# now
z=`date +%y%m%d`
#echo "Stat: $x; Now: $z; File: $d; $m $str ago: $e"
# checks whether file is more recent then req
if [ $d -ge $e -a $d -le $z ] # ToDo: fix year wrap-arounds
then
# be verbose if we found a recent file
if [ $verb -eq 1 ]
then echo "$f: modified (mmdd) $d"
fi
# do the line count
l=`wc -l $f | sed 's/^\([0-9]*\).*$/\1/'`
echo "$f: $l"
# increase the counters
n=$[ $n + 1 ]
s=$[ $s + $l ]
else
# not strictly necessary, because it's the end of the loop
continue
fi
done
echo "$n files in total, with $s lines in total"
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| Exercise | |
|---|---|---|
Extend Version 7 of the line count example above, to also compute the total number of bytes and the total number of words in the input file.
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