Advanced Bash-Scripting Guide: A complete guide to shell scripting
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12.4. Text Processing Commands

Command Listing

sort

File sorter, often used as a filter in a pipe. This command sorts a text stream or file forwards or backwards, or according to various keys or character positions. Using the -m option, it merges presorted input files. The info page lists its many capabilities and options. See Example 10-8 and Example 10-9.

tsort

Topological sort, reading in pairs of whitespace-separated strings and sorting according to input patterns.

diff, patch

diff: flexible file comparison utility. It compares the target files line-by-line sequentially. In some applications, such as comparing word dictionaries, it may be helpful to filter the files through sort and uniq before piping them to diff. diff file-1 file-2 outputs the lines in the files that differ, with carets showing which file each particular line belongs to.

The --side-by-side option to diff outputs each compared file, line by line, in separate columns, with non-matching lines marked.

There are available various fancy frontends for diff, such as spiff, wdiff, xdiff, and mgdiff.

The diff command returns an exit status of 0 if the compared files are identical, and 1 if they differ. This permits use of diff in a test construct within a shell script (see below).

A common use for diff is generating difference files to be used with patch The -e option outputs files suitable for ed or ex scripts.

patch: flexible versioning utility. Given a difference file generated by diff, patch can upgrade a previous version of a package to a newer version. It is much more convenient to distribute a relatively small "diff" file than the entire body of a newly revised package. Kernel "patches" have become the preferred method of distributing the frequent releases of the Linux kernel.

patch -p1 <patch-file
# Takes all the changes listed in 'patch-file'
# and applies them to the files referenced therein.
# This upgrades to a newer version of the package.

Patching the kernel:

cd /usr/src
gzip -cd patchXX.gz | patch -p0
# Upgrading kernel source using 'patch'.
# From the Linux kernel docs "README",
# by anonymous author (Alan Cox?).

The diff command can also recursively compare directories (for the filenames present).

bash$ diff -r ~/notes1 ~/notes2
Only in /home/bozo/notes1: file02
 Only in /home/bozo/notes1: file03
 Only in /home/bozo/notes2: file04

Use zdiff to compare gzipped files.

diff3

An extended version of diff that compares three files at a time. This command returns an exit value of 0 upon successful execution, but unfortunately this gives no information about the results of the comparison.

bash$ diff3 file-1 file-2 file-3
====
 1:1c
   This is line 1 of "file-1".
 2:1c
   This is line 1 of "file-2".
 3:1c
   This is line 1 of "file-3"

sdiff

Compare and/or edit two files in order to merge them into an output file. Because of its interactive nature, this command would find little use in a script.

cmp

The cmp command is a simpler version of diff, above. Whereas diff reports the differences between two files, cmp merely shows at what point they differ.

Like diff, cmp returns an exit status of 0 if the compared files are identical, and 1 if they differ. This permits use in a test construct within a shell script.

Example 12-8. Using cmp to compare two files within a script.

#!/bin/bash

ARGS=2  # Two args to script expected.
E_BADARGS=65

if [ $# -ne "$ARGS" ]
then
  echo "Usage: `basename $0` file1 file2"
  exit $E_BADARGS
fi


cmp $1 $2 > /dev/null  # /dev/null buries the output of the "cmp" command.
# Also works with 'diff', i.e.,   diff $1 $2 > /dev/null

if [ $? -eq 0 ]        # Test exit status of "cmp" command.
then
  echo "File \"$1\" is identical to file \"$2\"."
else  
  echo "File \"$1\" differs from file \"$2\"."
fi

exit 0

Use zcmp on gzipped files.

comm

Versatile file comparison utility. The files must be sorted for this to be useful.

comm -options first-file second-file

comm file-1 file-2 outputs three columns:

column 1 = lines unique to file-1
column 2 = lines unique to file-2
column 3 = lines common to both.

The options allow suppressing output of one or more columns.

-1 suppresses column 1
-2 suppresses column 2
-3 suppresses column 3
-12 suppresses both columns 1 and 2, etc.

uniq

This filter removes duplicate lines from a sorted file. It is often seen in a pipe coupled with sort.

cat list-1 list-2 list-3 | sort | uniq > final.list
# Concatenates the list files,
# sorts them,
# removes duplicate lines,
# and finally writes the result to an output file.

The useful -c option prefixes each line of the input file with its number of occurrences.

bash$ cat testfile
This line occurs only once.
 This line occurs twice.
 This line occurs twice.
 This line occurs three times.
 This line occurs three times.
 This line occurs three times.


bash$ uniq -c testfile
      1 This line occurs only once.
       2 This line occurs twice.
       3 This line occurs three times.


bash$ sort testfile | uniq -c | sort -nr
      3 This line occurs three times.
       2 This line occurs twice.
       1 This line occurs only once.

The sort INPUTFILE | uniq -c | sort -nr command string produces a frequency of occurrence listing on the INPUTFILE file (the -nr options to sort cause a reverse numerical sort). This template finds use in analysis of log files and dictionary lists, and wherever the lexical structure of a document needs to be examined.

Example 12-9. Word Frequency Analysis

#!/bin/bash
# wf.sh: Crude word frequency analysis on a text file.


# Check for input file on command line.
ARGS=1
E_BADARGS=65
E_NOFILE=66

if [ $# -ne $ARGS ]  # Correct number of arguments passed to script?
then
  echo "Usage: `basename $0` filename"
  exit $E_BADARGS
fi

if [ -f "$1" ]       # Check if file exists.
then
  file_name=$1
else
  echo "File \"$1\" does not exist."
  exit $E_NOFILE
fi



########################################################
# main
sed -e 's/\.//g'  -e 's/ /\
/g' "$1" | tr 'A-Z' 'a-z' | sort | uniq -c | sort -nr
#                           =========================
#                            Frequency of occurrence

#  Filter out periods and
#+ change space between words to linefeed,
#+ then shift characters to lowercase, and
#+ finally prefix occurrence count and sort numerically.
########################################################

# Exercises for the reader:
# 1) Add 'sed' commands to filter out other punctuation, such as commas.
# 2) Modify to also filter out multiple spaces and other whitespace.
# 3) Add a secondary sort key, so that instances of equal occurrence
#+   are sorted alphabetically.

exit 0

bash$ cat testfile
This line occurs only once.
 This line occurs twice.
 This line occurs twice.
 This line occurs three times.
 This line occurs three times.
 This line occurs three times.


bash$ ./wf.sh testfile
      6 this
       6 occurs
       6 line
       3 times
       3 three
       2 twice
       1 only
       1 once

expand, unexpand

The expand filter converts tabs to spaces. It is often used in a pipe.

The unexpand filter converts spaces to tabs. This reverses the effect of expand.

cut

A tool for extracting fields from files. It is similar to the print $N command set in awk, but more limited. It may be simpler to use cut in a script than awk. Particularly important are the -d (delimiter) and -f (field specifier) options.

Using cut to obtain a listing of the mounted filesystems:

cat /etc/mtab | cut -d ' ' -f1,2

Using cut to list the OS and kernel version:

uname -a | cut -d" " -f1,3,11,12

Using cut to extract message headers from an e-mail folder:

bash$ grep '^Subject:' read-messages | cut -c10-80
Re: Linux suitable for mission-critical apps?
 MAKE MILLIONS WORKING AT HOME!!!
 Spam complaint
 Re: Spam complaint

Using cut to parse a file:

# List all the users in /etc/passwd.

FILENAME=/etc/passwd

for user in $(cut -d: -f1 $FILENAME)
do
  echo $user
done

# Thanks, Oleg Philon for suggesting this.

cut -d ' ' -f2,3 filename is equivalent to awk -F'[ ]' '{ print $2, $3 }' filename

See also Example 12-27.

tail

lists the end of a file to stdout (the default is 10 lines). Commonly used to keep track of changes to a system logfile, using the -f option, which outputs lines appended to the file.

Example 12-11. Using tail to monitor the system log

#!/bin/bash

filename=sys.log

cat /dev/null > $filename; echo "Creating / cleaning out file."
# Creates file if it does not already exist,
# and truncates it to zero length if it does.
# : > filename   also works.

tail /var/log/messages > $filename  
# /var/log/messages must have world read permission for this to work.

echo "$filename contains tail end of system log."

exit 0

See also Example 12-4, Example 12-27 and Example 30-5.

grep

A multi-purpose file search tool that uses regular expressions. It was originally a command/filter in the venerable ed line editor, g/re/p, that is, global - regular expression - print.

grep pattern [file...]

Search the target file(s) for occurrences of pattern, where pattern may be literal text or a regular expression.

bash$ grep '[rst]ystem.$' osinfo.txt
The GPL governs the distribution of the Linux operating system.

If no target file(s) specified, grep works as a filter on stdout, as in a pipe.

bash$ ps ax | grep clock
765 tty1     S      0:00 xclock
 901 pts/1    S      0:00 grep clock

The -i option causes a case-insensitive search.

The -l option lists only the files in which matches were found, but not the matching lines.

The -n option lists the matching lines, together with line numbers.

bash$ grep -n Linux osinfo.txt
2:This is a file containing information about Linux.
 6:The GPL governs the distribution of the Linux operating system.

The -v (or --invert-match) option filters out matches.

grep pattern1 *.txt | grep -v pattern2

# Matches all lines in "*.txt" files containing "pattern1",
# but ***not*** "pattern2".

The -c (--count) option gives a numerical count of matches, rather than actually listing the matches.

grep -c txt *.sgml   # (number of occurrences of "txt" in "*.sgml" files)


#   grep -cz .
#            ^ dot
# means count (-c) zero-separated (-z) items matching "."
# that is, non-empty ones (containing at least 1 character).
# 
printf 'a b\nc  d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz .     # 4
printf 'a b\nc  d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz '$'   # 5
printf 'a b\nc  d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz '^'   # 5
#
printf 'a b\nc  d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -c '$'    # 9
# By default, newline chars (\n) separate items to match. 

# Note that the -z option is GNU "grep" specific.


# Thanks, S.C.

When invoked with more than one target file given, grep specifies which file contains matches.

bash$ grep Linux osinfo.txt misc.txt
osinfo.txt:This is a file containing information about Linux.
 osinfo.txt:The GPL governs the distribution of the Linux operating system.
 misc.txt:The Linux operating system is steadily gaining in popularity.

To force grep to show the filename when searching only one target file, simply give /dev/null as the second file.

bash$ grep Linux osinfo.txt /dev/null
osinfo.txt:This is a file containing information about Linux.
 osinfo.txt:The GPL governs the distribution of the Linux operating system.

If there is a successful match, grep returns an exit status of 0, which makes it useful in a condition test in a script, especially in combination with the -q option to suppress output.

SUCCESS=0                      # if grep lookup succeeds
word=Linux
filename=data.file

grep -q "$word" "$filename"    # The "-q" option causes nothing to echo to stdout.

if [ $? -eq $SUCCESS ]
then
  echo "$word found in $filename"
else
  echo "$word not found in $filename"
fi

Example 30-5 demonstrates how to use grep to search for a word pattern in a system logfile.

Example 12-12. Emulating "grep" in a script

#!/bin/bash
# grp.sh: Very crude reimplementation of 'grep'.

E_BADARGS=65

if [ -z "$1" ]    # Check for argument to script.
then
  echo "Usage: `basename $0` pattern"
  exit $E_BADARGS
fi  

echo

for file in *     # Traverse all files in $PWD.
do
  output=$(sed -n /"$1"/p $file)  # Command substitution.

  if [ ! -z "$output" ]           # What happens if "$output" is not quoted?
  then
    echo -n "$file: "
    echo $output
  fi              #  sed -ne "/$1/s|^|${file}: |p"  is equivalent to above.

  echo
done  

echo

exit 0

# Exercises for reader:
# --------------------
# 1) Add newlines to output, if more than one match in any given file.
# 2) Add features.

egrep is the same as grep -E. This uses a somewhat different, extended set of regular expressions, which can make the search somewhat more flexible.

fgrep is the same as grep -F. It does a literal string search (no regular expressions), which allegedly speeds things up a bit.

agrep extends the capabilities of grep to approximate matching. The search string may differ by a specified number of characters from the resulting matches. This utility is not part of the core Linux distribution.

To search compressed files, use zgrep, zegrep, or zfgrep. These also work on non-compressed files, though slower than plain grep, egrep, fgrep. They are handy for searching through a mixed set of files, some compressed, some not.

To search bzipped files, use bzgrep.

look

The command look works like grep, but does a lookup on a "dictionary", a sorted word list. By default, look searches for a match in /usr/dict/words, but a different dictionary file may be specified.

Example 12-13. Checking words in a list for validity

#!/bin/bash
# lookup: Does a dictionary lookup on each word in a data file.

file=words.data  # Data file from which to read words to test.

echo

while [ "$word" != end ]  # Last word in data file.
do
  read word      # From data file, because of redirection at end of loop.
  look $word > /dev/null  # Don't want to display lines in dictionary file.
  lookup=$?      # Exit status of 'look' command.

  if [ "$lookup" -eq 0 ]
  then
    echo "\"$word\" is valid."
  else
    echo "\"$word\" is invalid."
  fi  

done <"$file"    # Redirects stdin to $file, so "reads" come from there.

echo

exit 0

# ----------------------------------------------------------------
# Code below line will not execute because of "exit" command above.


# Stephane Chazelas proposes the following, more concise alternative:

while read word && [[ $word != end ]]
do if look "$word" > /dev/null
   then echo "\"$word\" is valid."
   else echo "\"$word\" is invalid."
   fi
done <"$file"

exit 0

sed, awk

Scripting languages especially suited for parsing text files and command output. May be embedded singly or in combination in pipes and shell scripts.

sed

Non-interactive "stream editor", permits using many ex commands in batch mode. It finds many uses in shell scripts.

awk

Programmable file extractor and formatter, good for manipulating and/or extracting fields (columns) in structured text files. Its syntax is similar to C.

wc

wc gives a "word count" on a file or I/O stream:

bash $ wc /usr/doc/sed-3.02/README
20     127     838 /usr/doc/sed-3.02/README
[20 lines  127 words  838 characters]

wc -w gives only the word count.

wc -l gives only the line count.

wc -c gives only the character count.

wc -L gives only the length of the longest line.

Using wc to count how many .txt files are in current working directory:

$ ls *.txt | wc -l
# Will work as long as none of the "*.txt" files have a linefeed in their name.

# Alternative ways of doing this are:
#      find . -maxdepth 1 -name \*.txt -print0 | grep -cz .
#      (shopt -s nullglob; set -- *.txt; echo $#)

# Thanks, S.C.

Using wc to total up the size of all the files whose names begin with letters in the range d - h

bash$ wc [d-h]* | grep total | awk '{print $3}'
71832

Using wc to count the instances of the word "Linux" in the main source file for this book.

bash$ grep Linux abs-book.sgml | wc -l
50

See also Example 12-27 and Example 16-5.

Certain commands include some of the functionality of wc as options.

... | grep foo | wc -l
# This frequently used construct can be more concisely rendered.

... | grep -c foo
# Just use the "-c" (or "--count") option of grep.

# Thanks, S.C.

tr

character translation filter.

Must use quoting and/or brackets, as appropriate. Quotes prevent the shell from reinterpreting the special characters in tr command sequences. Brackets should be quoted to prevent expansion by the shell.

Either tr "A-Z" "*" <filename or tr A-Z \* <filename changes all the uppercase letters in filename to asterisks (writes to stdout). On some systems this may not work, but tr A-Z '[**]' will.

The -d option deletes a range of characters.

tr -d 0-9 <filename
# Deletes all digits from the file "filename".

The --squeeze-repeats (or -s) option deletes all but the first instance of a string of consecutive characters. This option is useful for removing excess whitespace.

bash$ echo "XXXXX" | tr --squeeze-repeats 'X'
X

Example 12-14. toupper: Transforms a file to all uppercase.

#!/bin/bash
# Changes a file to all uppercase.

E_BADARGS=65

if [ -z "$1" ]  # Standard check for command line arg.
then
  echo "Usage: `basename $0` filename"
  exit $E_BADARGS
fi  

tr a-z A-Z <"$1"

# Same effect as above, but using POSIX character set notation:
#        tr '[:lower:]' '[:upper:]' <"$1"
# Thanks, S.C.

exit 0

Example 12-15. lowercase: Changes all filenames in working directory to lowercase.

#! /bin/bash
#
# Changes every filename in working directory to all lowercase.
#
# Inspired by a script of John Dubois,
# which was translated into into Bash by Chet Ramey,
# and considerably simplified by Mendel Cooper, author of this document.


for filename in *                # Traverse all files in directory.
do
   fname=`basename $filename`
   n=`echo $fname | tr A-Z a-z`  # Change name to lowercase.
   if [ "$fname" != "$n" ]       # Rename only files not already lowercase.
   then
     mv $fname $n
   fi  
done   

exit 0


# Code below this line will not execute because of "exit".
#--------------------------------------------------------#
# To run it, delete script above line.

# The above script will not work on filenames containing blanks or newlines.

# Stephane Chazelas therefore suggests the following alternative:


for filename in *    # Not necessary to use basename,
                     # since "*" won't return any file containing "/".
do n=`echo "$filename/" | tr '[:upper:]' '[:lower:]'`
#                             POSIX char set notation.
#                    Slash added so that trailing newlines are not
#                    removed by command substitution.
   # Variable substitution:
   n=${n%/}          # Removes trailing slash, added above, from filename.
   [[ $filename == $n ]] || mv "$filename" "$n"
                     # Checks if filename already lowercase.
done

exit 0

Example 12-16. du: DOS to UNIX text file conversion.

#!/bin/bash
# du.sh: DOS to UNIX text file converter.

E_WRONGARGS=65

if [ -z "$1" ]
then
  echo "Usage: `basename $0` filename-to-convert"
  exit $E_WRONGARGS
fi

NEWFILENAME=$1.unx

CR='\015'  # Carriage return.
# Lines in a DOS text file end in a CR-LF.

tr -d $CR < $1 > $NEWFILENAME
# Delete CR and write to new file.

echo "Original DOS text file is \"$1\"."
echo "Converted UNIX text file is \"$NEWFILENAME\"."

exit 0

Example 12-17. rot13: rot13, ultra-weak encryption.

#!/bin/bash
# rot13.sh: Classic rot13 algorithm, encryption that might fool a 3-year old.

# Usage: ./rot13.sh filename
# or     ./rot13.sh <filename
# or     ./rot13.sh and supply keyboard input (stdin)

cat "$@" | tr 'a-zA-Z' 'n-za-mN-ZA-M'   # "a" goes to "n", "b" to "o", etc.
# The 'cat "$@"' construction
# permits getting input either from stdin or from files.

exit 0

Example 12-18. Generating "Crypto-Quote" Puzzles

#!/bin/bash
# crypto-quote.sh: Encrypt quotes

# Will encrypt famous quotes in a simple monoalphabetic substitution.
#  The result is similar to the "Crypto Quote" puzzles
#+ seen in the Op Ed pages of the Sunday paper.


key=ETAOINSHRDLUBCFGJMQPVWZYXK
# The "key" is nothing more than a scrambled alphabet.
# Changing the "key" changes the encryption.

# The 'cat "$@"' construction gets input either from stdin or from files.
# If using stdin, terminate input with a Control-D.
# Otherwise, specify filename as command-line parameter.

cat "$@" | tr "a-z" "A-Z" | tr "A-Z" "$key"
#        |  to uppercase  |     encrypt       
# Will work on lowercase, uppercase, or mixed-case quotes.
# Passes non-alphabetic characters through unchanged.


# Try this script with something like
# "Nothing so needs reforming as other people's habits."
# --Mark Twain
#
# Output is:
# "CFPHRCS QF CIIOQ MINFMBRCS EQ FPHIM GIFGUI'Q HETRPQ."
# --BEML PZERC

# To reverse the encryption:
# cat "$@" | tr "$key" "A-Z"


#  This simple-minded cipher can be broken by an average 12-year old
#+ using only pencil and paper.

exit 0

fold

A filter that wraps lines of input to a specified width. This is especially useful with the -s option, which breaks lines at word spaces (see Example 12-19 and Example A-2).

fmt

Simple-minded file formatter, used as a filter in a pipe to "wrap" long lines of text output.

Example 12-19. Formatted file listing.

#!/bin/bash

WIDTH=40                    # 40 columns wide.

b=`ls /usr/local/bin`       # Get a file listing...

echo $b | fmt -w $WIDTH

# Could also have been done by
#  echo $b | fold - -s -w $WIDTH
 
exit 0

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