Case Study File Example Linux

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 , 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 , by echoing the value of the variable 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 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<&-

As documented at the start of the script, it is called like this (you must have a file in your current directory):

./line_count.sh text_file.txt
Sample text file
 

You can get a sizable sample text file by typing:

cp /home/msc/public/LinuxIntro/WaD.txt text_file.txt

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"

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 .

#!/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"

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"

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"

Version 5: Using an array to store all line counts

The example below uses shell arrays to store all filenames () and its number of lines (). The elements in an array are referred to using the usual [ ] notation, e.g. refers to the first element in the array . 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"

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"

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 shell function is used to iterate over all options (given in the following string) and assigning the current option to variable . 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, 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"
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.

./loc8.sh text_file.txt loc8.sh 2 files in total, with 1494438 bytes, 746 words, 27846 lines in total
Hint: check the man page for wc.

Да, - в сердцах бросил Джабба.  - Шифр-убийца. Но единственный человек, которому известен ключ, мертв. - А метод грубой силы? - предложил Бринкерхофф.  - Можно ли с его помощью найти ключ.

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