15.5. File and Archiving Commands



The standard UNIX archiving utility. [1] Originally a Tape ARchiving program, it has developed into a general purpose package that can handle all manner of archiving with all types of destination devices, ranging from tape drives to regular files to even stdout (see Example 3-4). GNU tar has been patched to accept various compression filters, for example: tar czvf archive_name.tar.gz *, which recursively archives and gzips all files in a directory tree except dotfiles in the current working directory ($PWD). [2]

Some useful tar options:

  1. -c create (a new archive)

  2. -x extract (files from existing archive)

  3. --delete delete (files from existing archive)


    This option will not work on magnetic tape devices.

  4. -r append (files to existing archive)

  5. -A append (tar files to existing archive)

  6. -t list (contents of existing archive)

  7. -u update archive

  8. -d compare archive with specified filesystem

  9. -z gzip the archive

    (compress or uncompress, depending on whether combined with the -c or -x) option

  10. -j bzip2 the archive


It may be difficult to recover data from a corrupted gzipped tar archive. When archiving important files, make multiple backups.


Shell archiving utility. The files in a shell archive are concatenated without compression, and the resultant archive is essentially a shell script, complete with #!/bin/sh header, and containing all the necessary unarchiving commands. Shar archives still show up in Usenet newsgroups, but otherwise shar has been pretty well replaced by tar/gzip. The unshar command unpacks shar archives.


Creation and manipulation utility for archives, mainly used for binary object file libraries.


The Red Hat Package Manager, or rpm utility provides a wrapper for source or binary archives. It includes commands for installing and checking the integrity of packages, among other things.

A simple rpm -i package_name.rpm usually suffices to install a package, though there are many more options available.


rpm -qf identifies which package a file originates from.

 bash$ rpm -qf /bin/ls


rpm -qa gives a complete list of all installed rpm packages on a given system. An rpm -qa package_name lists only the package(s) corresponding to package_name.

 bash$ rpm -qa
 bash$ rpm -qa docbook-utils
 bash$ rpm -qa docbook | grep docbook


This specialized archiving copy command (copy input and output) is rarely seen any more, having been supplanted by tar/gzip. It still has its uses, such as moving a directory tree. With an appropriate block size (for copying) specified, it can be appreciably faster than tar.

Example 15-28. Using cpio to move a directory tree

   1 #!/bin/bash
   3 # Copying a directory tree using cpio.
   5 # Advantages of using 'cpio':
   6 #   Speed of copying. It's faster than 'tar' with pipes.
   7 #   Well suited for copying special files (named pipes, etc.)
   8 #+  that 'cp' may choke on.
  10 ARGS=2
  11 E_BADARGS=65
  13 if [ $# -ne "$ARGS" ]
  14 then
  15   echo "Usage: `basename $0` source destination"
  16   exit $E_BADARGS
  17 fi  
  19 source="$1"
  20 destination="$2"
  22 ###################################################################
  23 find "$source" -depth | cpio -admvp "$destination"
  24 #               ^^^^^         ^^^^^
  25 #  Read the 'find' and 'cpio' info pages to decipher these options.
  26 #  The above works only relative to $PWD (current directory) . . .
  27 #+ full pathnames are specified.
  28 ###################################################################
  31 # Exercise:
  32 # --------
  34 #  Add code to check the exit status ($?) of the 'find | cpio' pipe
  35 #+ and output appropriate error messages if anything went wrong.
  37 exit $?


This command extracts a cpio archive from an rpm one.

Example 15-29. Unpacking an rpm archive

   1 #!/bin/bash
   2 # de-rpm.sh: Unpack an 'rpm' archive
   4 : ${1?"Usage: `basename $0` target-file"}
   5 # Must specify 'rpm' archive name as an argument.
   8 TEMPFILE=$$.cpio                         #  Tempfile with "unique" name.
   9                                          #  $$ is process ID of script.
  11 rpm2cpio < $1 > $TEMPFILE                #  Converts rpm archive into
  12                                          #+ cpio archive.
  13 cpio --make-directories -F $TEMPFILE -i  #  Unpacks cpio archive.
  14 rm -f $TEMPFILE                          #  Deletes cpio archive.
  16 exit 0
  18 #  Exercise:
  19 #  Add check for whether 1) "target-file" exists and
  20 #+                       2) it is an rpm archive.
  21 #  Hint:                    Parse output of 'file' command.



The standard GNU/UNIX compression utility, replacing the inferior and proprietary compress. The corresponding decompression command is gunzip, which is the equivalent of gzip -d.


The -c option sends the output of gzip to stdout. This is useful when piping to other commands.

The zcat filter decompresses a gzipped file to stdout, as possible input to a pipe or redirection. This is, in effect, a cat command that works on compressed files (including files processed with the older compress utility). The zcat command is equivalent to gzip -dc.


On some commercial UNIX systems, zcat is a synonym for uncompress -c, and will not work on gzipped files.

See also Example 7-7.


An alternate compression utility, usually more efficient (but slower) than gzip, especially on large files. The corresponding decompression command is bunzip2.


Newer versions of tar have been patched with bzip2 support.

compress, uncompress

This is an older, proprietary compression utility found in commercial UNIX distributions. The more efficient gzip has largely replaced it. Linux distributions generally include a compress workalike for compatibility, although gunzip can unarchive files treated with compress.


The znew command transforms compressed files into gzipped ones.


Yet another compression (squeeze) utility, a filter that works only on sorted ASCII word lists. It uses the standard invocation syntax for a filter, sq < input-file > output-file. Fast, but not nearly as efficient as gzip. The corresponding uncompression filter is unsq, invoked like sq.


The output of sq may be piped to gzip for further compression.

zip, unzip

Cross-platform file archiving and compression utility compatible with DOS pkzip.exe. "Zipped" archives seem to be a more common medium of file exchange on the Internet than "tarballs."

unarc, unarj, unrar

These Linux utilities permit unpacking archives compressed with the DOS arc.exe, arj.exe, and rar.exe programs.

File Information


A utility for identifying file types. The command file file-name will return a file specification for file-name, such as ascii text or data. It references the magic numbers found in /usr/share/magic, /etc/magic, or /usr/lib/magic, depending on the Linux/UNIX distribution.

The -f option causes file to run in batch mode, to read from a designated file a list of filenames to analyze. The -z option, when used on a compressed target file, forces an attempt to analyze the uncompressed file type.

 bash$ file test.tar.gz
 test.tar.gz: gzip compressed data, deflated,
 last modified: Sun Sep 16 13:34:51 2001, os: Unix
 bash file -z test.tar.gz
 test.tar.gz: GNU tar archive (gzip compressed data, deflated,
 last modified: Sun Sep 16 13:34:51 2001, os: Unix)

   1 # Find sh and Bash scripts in a given directory:
   3 DIRECTORY=/usr/local/bin
   4 KEYWORD=Bourne
   5 # Bourne and Bourne-Again shell scripts
   7 file $DIRECTORY/* | fgrep $KEYWORD
   9 # Output:
  11 # /usr/local/bin/burn-cd:          Bourne-Again shell script text executable
  12 # /usr/local/bin/burnit:           Bourne-Again shell script text executable
  13 # /usr/local/bin/cassette.sh:      Bourne shell script text executable
  14 # /usr/local/bin/copy-cd:          Bourne-Again shell script text executable
  15 # . . .

Example 15-30. Stripping comments from C program files

   1 #!/bin/bash
   2 # strip-comment.sh: Strips out the comments (/* COMMENT */) in a C program.
   4 E_NOARGS=0
   5 E_ARGERROR=66
   8 if [ $# -eq "$E_NOARGS" ]
   9 then
  10   echo "Usage: `basename $0` C-program-file" >&2 # Error message to stderr.
  11   exit $E_ARGERROR
  12 fi  
  14 # Test for correct file type.
  15 type=`file $1 | awk '{ print $2, $3, $4, $5 }'`
  16 # "file $1" echoes file type . . .
  17 # Then awk removes the first field, the filename . . .
  18 # Then the result is fed into the variable "type."
  19 correct_type="ASCII C program text"
  21 if [ "$type" != "$correct_type" ]
  22 then
  23   echo
  24   echo "This script works on C program files only."
  25   echo
  26   exit $E_WRONG_FILE_TYPE
  27 fi  
  30 # Rather cryptic sed script:
  31 #--------
  32 sed '
  33 /^\/\*/d
  34 /.*\*\//d
  35 ' $1
  36 #--------
  37 # Easy to understand if you take several hours to learn sed fundamentals.
  40 #  Need to add one more line to the sed script to deal with
  41 #+ case where line of code has a comment following it on same line.
  42 #  This is left as a non-trivial exercise.
  44 #  Also, the above code deletes non-comment lines with a "*/" . . .
  45 #+ not a desirable result.
  47 exit 0
  50 # ----------------------------------------------------------------
  51 # Code below this line will not execute because of 'exit 0' above.
  53 # Stephane Chazelas suggests the following alternative:
  55 usage() {
  56   echo "Usage: `basename $0` C-program-file" >&2
  57   exit 1
  58 }
  60 WEIRD=`echo -n -e '\377'`   # or WEIRD=$'\377'
  61 [[ $# -eq 1 ]] || usage
  62 case `file "$1"` in
  63   *"C program text"*) sed -e "s%/\*%${WEIRD}%g;s%\*/%${WEIRD}%g" "$1" \
  64      | tr '\377\n' '\n\377' \
  65      | sed -ne 'p;n' \
  66      | tr -d '\n' | tr '\377' '\n';;
  67   *) usage;;
  68 esac
  70 #  This is still fooled by things like:
  71 #  printf("/*");
  72 #  or
  73 #  /*  /* buggy embedded comment */
  74 #
  75 #  To handle all special cases (comments in strings, comments in string
  76 #+ where there is a \", \\" ...),
  77 #+ the only way is to write a C parser (using lex or yacc perhaps?).
  79 exit 0


which command gives the full path to "command." This is useful for finding out whether a particular command or utility is installed on the system.

$bash which rm

For an interesting use of this command, see Example 33-14.


Similar to which, above, whereis command gives the full path to "command," but also to its manpage.

$bash whereis rm
 rm: /bin/rm /usr/share/man/man1/rm.1.bz2


whatis command looks up "command" in the whatis database. This is useful for identifying system commands and important configuration files. Consider it a simplified man command.

$bash whatis whatis
 whatis               (1)  - search the whatis database for complete words

Example 15-31. Exploring /usr/X11R6/bin

   1 #!/bin/bash
   3 # What are all those mysterious binaries in /usr/X11R6/bin?
   5 DIRECTORY="/usr/X11R6/bin"
   6 # Try also "/bin", "/usr/bin", "/usr/local/bin", etc.
   8 for file in $DIRECTORY/*
   9 do
  10   whatis `basename $file`   # Echoes info about the binary.
  11 done
  13 exit 0
  15 # You may wish to redirect output of this script, like so:
  16 # ./what.sh >>whatis.db
  17 # or view it a page at a time on stdout,
  18 # ./what.sh | less

See also Example 10-3.


Show a detailed directory listing. The effect is similar to ls -lb.

This is one of the GNU fileutils.

 bash$ vdir
 total 10
 -rw-r--r--    1 bozo  bozo      4034 Jul 18 22:04 data1.xrolo
 -rw-r--r--    1 bozo  bozo      4602 May 25 13:58 data1.xrolo.bak
 -rw-r--r--    1 bozo  bozo       877 Dec 17  2000 employment.xrolo
 bash ls -l
 total 10
 -rw-r--r--    1 bozo  bozo      4034 Jul 18 22:04 data1.xrolo
 -rw-r--r--    1 bozo  bozo      4602 May 25 13:58 data1.xrolo.bak
 -rw-r--r--    1 bozo  bozo       877 Dec 17  2000 employment.xrolo

locate, slocate

The locate command searches for files using a database stored for just that purpose. The slocate command is the secure version of locate (which may be aliased to slocate).

$bash locate hickson


Disclose the file that a symbolic link points to.

 bash$ readlink /usr/bin/awk


Use the strings command to find printable strings in a binary or data file. It will list sequences of printable characters found in the target file. This might be handy for a quick 'n dirty examination of a core dump or for looking at an unknown graphic image file (strings image-file | more might show something like JFIF, which would identify the file as a jpeg graphic). In a script, you would probably parse the output of strings with grep or sed. See Example 10-7 and Example 10-9.

Example 15-32. An "improved" strings command

   1 #!/bin/bash
   2 # wstrings.sh: "word-strings" (enhanced "strings" command)
   3 #
   4 #  This script filters the output of "strings" by checking it
   5 #+ against a standard word list file.
   6 #  This effectively eliminates gibberish and noise,
   7 #+ and outputs only recognized words.
   9 # ===========================================================
  10 #                 Standard Check for Script Argument(s)
  11 ARGS=1
  12 E_BADARGS=65
  13 E_NOFILE=66
  15 if [ $# -ne $ARGS ]
  16 then
  17   echo "Usage: `basename $0` filename"
  18   exit $E_BADARGS
  19 fi
  21 if [ ! -f "$1" ]                      # Check if file exists.
  22 then
  23     echo "File \"$1\" does not exist."
  24     exit $E_NOFILE
  25 fi
  26 # ===========================================================
  29 MINSTRLEN=3                           #  Minimum string length.
  30 WORDFILE=/usr/share/dict/linux.words  #  Dictionary file.
  31                                       #  May specify a different
  32                                       #+ word list file
  33                                       #+ of one-word-per-line format.
  36 wlist=`strings "$1" | tr A-Z a-z | tr '[:space:]' Z | \
  37 tr -cs '[:alpha:]' Z | tr -s '\173-\377' Z | tr Z ' '`
  39 # Translate output of 'strings' command with multiple passes of 'tr'.
  40 #  "tr A-Z a-z"  converts to lowercase.
  41 #  "tr '[:space:]'"  converts whitespace characters to Z's.
  42 #  "tr -cs '[:alpha:]' Z"  converts non-alphabetic characters to Z's,
  43 #+ and squeezes multiple consecutive Z's.
  44 #  "tr -s '\173-\377' Z"  converts all characters past 'z' to Z's
  45 #+ and squeezes multiple consecutive Z's,
  46 #+ which gets rid of all the weird characters that the previous
  47 #+ translation failed to deal with.
  48 #  Finally, "tr Z ' '" converts all those Z's to whitespace,
  49 #+ which will be seen as word separators in the loop below.
  51 #  ****************************************************************
  52 #  Note the technique of feeding the output of 'tr' back to itself,
  53 #+ but with different arguments and/or options on each pass.
  54 #  ****************************************************************
  57 for word in $wlist                    # Important:
  58                                       # $wlist must not be quoted here.
  59                                       # "$wlist" does not work.
  60                                       # Why not?
  61 do
  63   strlen=${#word}                     # String length.
  64   if [ "$strlen" -lt "$MINSTRLEN" ]   # Skip over short strings.
  65   then
  66     continue
  67   fi
  69   grep -Fw $word "$WORDFILE"          #  Match whole words only.
  70 #      ^^^                            #  "Fixed strings" and
  71                                       #+ "whole words" options. 
  73 done  
  76 exit $?


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. The -c and -u options likewise make the output of the command easier to interpret.

There are available various fancy frontends for diff, such as sdiff, 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.

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

Patching the kernel:

   1 cd /usr/src
   2 gzip -cd patchXX.gz | patch -p0
   3 # Upgrading kernel source using 'patch'.
   4 # From the Linux kernel docs "README",
   5 # 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.


Use diffstat to create a histogram (point-distribution graph) of output from diff.


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
   This is line 1 of "file-1".
   This is line 1 of "file-2".
   This is line 1 of "file-3"


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.


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 15-33. Using cmp to compare two files within a script.

   1 #!/bin/bash
   3 ARGS=2  # Two args to script expected.
   4 E_BADARGS=65
   7 if [ $# -ne "$ARGS" ]
   8 then
   9   echo "Usage: `basename $0` file1 file2"
  10   exit $E_BADARGS
  11 fi
  13 if [[ ! -r "$1" || ! -r "$2" ]]
  14 then
  15   echo "Both files to be compared must exist and be readable."
  16   exit $E_UNREADABLE
  17 fi
  19 cmp $1 $2 &> /dev/null  # /dev/null buries the output of the "cmp" command.
  20 #   cmp -s $1 $2  has same result ("-s" silent flag to "cmp")
  21 #   Thank you  Anders Gustavsson for pointing this out.
  22 #
  23 # Also works with 'diff', i.e.,   diff $1 $2 &> /dev/null
  25 if [ $? -eq 0 ]         # Test exit status of "cmp" command.
  26 then
  27   echo "File \"$1\" is identical to file \"$2\"."
  28 else  
  29   echo "File \"$1\" differs from file \"$2\"."
  30 fi
  32 exit 0


Use zcmp on gzipped files.


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.

This command is useful for comparing "dictionaries" or word lists -- sorted text files with one word per line.



Strips the path information from a file name, printing only the file name. The construction basename $0 lets the script know its name, that is, the name it was invoked by. This can be used for "usage" messages if, for example a script is called with missing arguments:
   1 echo "Usage: `basename $0` arg1 arg2 ... argn"


Strips the basename from a filename, printing only the path information.


basename and dirname can operate on any arbitrary string. The argument does not need to refer to an existing file, or even be a filename for that matter (see Example A-7).

Example 15-34. basename and dirname

   1 #!/bin/bash
   3 a=/home/bozo/daily-journal.txt
   5 echo "Basename of /home/bozo/daily-journal.txt = `basename $a`"
   6 echo "Dirname of /home/bozo/daily-journal.txt = `dirname $a`"
   7 echo
   8 echo "My own home is `basename ~/`."         # `basename ~` also works.
   9 echo "The home of my home is `dirname ~/`."  # `dirname ~`  also works.
  11 exit 0

split, csplit

These are utilities for splitting a file into smaller chunks. They are usually used for splitting up large files in order to back them up on floppies or preparatory to e-mailing or uploading them.

The csplit command splits a file according to context, the split occuring where patterns are matched.

Encoding and Encryption

sum, cksum, md5sum, sha1sum

These are utilities for generating checksums. A checksum is a number mathematically calculated from the contents of a file, for the purpose of checking its integrity. A script might refer to a list of checksums for security purposes, such as ensuring that the contents of key system files have not been altered or corrupted. For security applications, use the md5sum (message digest 5 checksum) command, or better yet, the newer sha1sum (Secure Hash Algorithm).

 bash$ cksum /boot/vmlinuz
 1670054224 804083 /boot/vmlinuz
 bash$ echo -n "Top Secret" | cksum
 3391003827 10
 bash$ md5sum /boot/vmlinuz
 0f43eccea8f09e0a0b2b5cf1dcf333ba  /boot/vmlinuz
 bash$ echo -n "Top Secret" | md5sum
 8babc97a6f62a4649716f4df8d61728f  -


The cksum command shows the size, in bytes, of its target, whether file or stdout.

The md5sum and sha1sum commands display a dash when they receive their input from stdout.

Example 15-35. Checking file integrity

   1 #!/bin/bash
   2 # file-integrity.sh: Checking whether files in a given directory
   3 #                    have been tampered with.
   6 E_BAD_DBFILE=71
   8 dbfile=File_record.md5
   9 # Filename for storing records (database file).
  12 set_up_database ()
  13 {
  14   echo ""$directory"" > "$dbfile"
  15   # Write directory name to first line of file.
  16   md5sum "$directory"/* >> "$dbfile"
  17   # Append md5 checksums and filenames.
  18 }
  20 check_database ()
  21 {
  22   local n=0
  23   local filename
  24   local checksum
  26   # ------------------------------------------- #
  27   #  This file check should be unnecessary,
  28   #+ but better safe than sorry.
  30   if [ ! -r "$dbfile" ]
  31   then
  32     echo "Unable to read checksum database file!"
  33     exit $E_BAD_DBFILE
  34   fi
  35   # ------------------------------------------- #
  37   while read record[n]
  38   do
  40     directory_checked="${record[0]}"
  41     if [ "$directory_checked" != "$directory" ]
  42     then
  43       echo "Directories do not match up!"
  44       # Tried to use file for a different directory.
  45       exit $E_DIR_NOMATCH
  46     fi
  48     if [ "$n" -gt 0 ]   # Not directory name.
  49     then
  50       filename[n]=$( echo ${record[$n]} | awk '{ print $2 }' )
  51       #  md5sum writes records backwards,
  52       #+ checksum first, then filename.
  53       checksum[n]=$( md5sum "${filename[n]}" )
  56       if [ "${record[n]}" = "${checksum[n]}" ]
  57       then
  58         echo "${filename[n]} unchanged."
  60       elif [ "`basename ${filename[n]}`" != "$dbfile" ]
  61              #  Skip over checksum database file,
  62              #+ as it will change with each invocation of script.
  63 	     #  ---
  64 	     #  This unfortunately means that when running
  65 	     #+ this script on $PWD, tampering with the
  66 	     #+ checksum database file will not be detected.
  67 	     #  Exercise: Fix this.
  68 	then
  69           echo "${filename[n]} : CHECKSUM ERROR!"
  70         # File has been changed since last checked.
  71       fi
  73       fi
  77     let "n+=1"
  78   done <"$dbfile"       # Read from checksum database file. 
  80 }  
  82 # =================================================== #
  83 # main ()
  85 if [ -z  "$1" ]
  86 then
  87   directory="$PWD"      #  If not specified,
  88 else                    #+ use current working directory.
  89   directory="$1"
  90 fi  
  92 clear                   # Clear screen.
  93 echo " Running file integrity check on $directory"
  94 echo
  96 # ------------------------------------------------------------------ #
  97   if [ ! -r "$dbfile" ] # Need to create database file?
  98   then
  99     echo "Setting up database file, \""$directory"/"$dbfile"\"."; echo
 100     set_up_database
 101   fi  
 102 # ------------------------------------------------------------------ #
 104 check_database          # Do the actual work.
 106 echo 
 108 #  You may wish to redirect the stdout of this script to a file,
 109 #+ especially if the directory checked has many files in it.
 111 exit 0
 113 #  For a much more thorough file integrity check,
 114 #+ consider the "Tripwire" package,
 115 #+ http://sourceforge.net/projects/tripwire/.

Also see Example A-19 and Example 33-14 for creative uses of the md5sum command.


There have been reports that the 128-bit md5sum can be cracked, so the more secure 160-bit sha1sum is a welcome new addition to the checksum toolkit.

Some security consultants think that even sha1sum can be compromised. So, what's next -- a 512-bit checksum utility?

 bash$ md5sum testfile
 e181e2c8720c60522c4c4c981108e367  testfile
 bash$ sha1sum testfile
 5d7425a9c08a66c3177f1e31286fa40986ffc996  testfile

Securely erase a file by overwriting it multiple times with random bit patterns before deleting it. This command has the same effect as Example 15-56, but does it in a more thorough and elegant manner.

This is one of the GNU fileutils.


Advanced forensic technology may still be able to recover the contents of a file, even after application of shred.


This utility encodes binary files (images, sound files, compressed files, etc.) into ASCII characters, making them suitable for transmission in the body of an e-mail message or in a newsgroup posting. This is especially useful where MIME (multimedia) encoding is not available.


This reverses the encoding, decoding uuencoded files back into the original binaries.

Example 15-36. Uudecoding encoded files

   1 #!/bin/bash
   2 # Uudecodes all uuencoded files in current working directory.
   4 lines=35        # Allow 35 lines for the header (very generous).
   6 for File in *   # Test all the files in $PWD.
   7 do
   8   search1=`head -n $lines $File | grep begin | wc -w`
   9   search2=`tail -n $lines $File | grep end | wc -w`
  10   #  Uuencoded files have a "begin" near the beginning,
  11   #+ and an "end" near the end.
  12   if [ "$search1" -gt 0 ]
  13   then
  14     if [ "$search2" -gt 0 ]
  15     then
  16       echo "uudecoding - $File -"
  17       uudecode $File
  18     fi  
  19   fi
  20 done  
  22 #  Note that running this script upon itself fools it
  23 #+ into thinking it is a uuencoded file,
  24 #+ because it contains both "begin" and "end".
  26 #  Exercise:
  27 #  --------
  28 #  Modify this script to check each file for a newsgroup header,
  29 #+ and skip to next if not found.
  31 exit 0


The fold -s command may be useful (possibly in a pipe) to process long uudecoded text messages downloaded from Usenet newsgroups.

mimencode, mmencode

The mimencode and mmencode commands process multimedia-encoded e-mail attachments. Although mail user agents (such as pine or kmail) normally handle this automatically, these particular utilities permit manipulating such attachments manually from the command line or in batch processing mode by means of a shell script.


At one time, this was the standard UNIX file encryption utility. [3] Politically motivated government regulations prohibiting the export of encryption software resulted in the disappearance of crypt from much of the UNIX world, and it is still missing from most Linux distributions. Fortunately, programmers have come up with a number of decent alternatives to it, among them the author's very own cruft (see Example A-4).



Create a temporary file [4] with a "unique" filename. When invoked from the command line without additional arguments, it creates a zero-length file in the /tmp directory.

 bash$ mktemp

   1 PREFIX=filename
   2 tempfile=`mktemp $PREFIX.XXXXXX`
   3 #                        ^^^^^^ Need at least 6 placeholders
   4 #+                              in the filename template.
   5 #   If no filename template supplied,
   6 #+ "tmp.XXXXXXXXXX" is the default.
   8 echo "tempfile name = $tempfile"
   9 # tempfile name = filename.QA2ZpY
  10 #                 or something similar...
  12 #  Creates a file of that name in the current working directory
  13 #+ with 600 file permissions.
  14 #  A "umask 177" is therefore unnecessary,
  15 #+ but it's good programming practice anyhow.


Utility for building and compiling binary packages. This can also be used for any set of operations that is triggered by incremental changes in source files.

The make command checks a Makefile, a list of file dependencies and operations to be carried out.

The make utility is, in effect, a powerful scripting language similar in many ways to Bash, but with the capability of recognizing dependencies. For in-depth coverage of this useful tool set, see the GNU software documentation site.


Special purpose file copying command, similar to cp, but capable of setting permissions and attributes of the copied files. This command seems tailormade for installing software packages, and as such it shows up frequently in Makefiles (in the make install : section). It could likewise find use in installation scripts.


This utility, written by Benjamin Lin and collaborators, converts DOS-formatted text files (lines terminated by CR-LF) to UNIX format (lines terminated by LF only), and vice-versa.


The ptx [targetfile] command outputs a permuted index (cross-reference list) of the targetfile. This may be further filtered and formatted in a pipe, if necessary.

more, less

Pagers that display a text file or stream to stdout, one screenful at a time. These may be used to filter the output of stdout . . . or of a script.

An interesting application of more is to "test drive" a command sequence, to forestall potentially unpleasant consequences.
   1 ls /home/bozo | awk '{print "rm -rf " $1}' | more
   2 #                                            ^^^^
   4 # Testing the effect of the following (disastrous) command line:
   5 #      ls /home/bozo | awk '{print "rm -rf " $1}' | sh
   6 #      Hand off to the shell to execute . . .       ^^



An archive, in the sense discussed here, is simply a set of related files stored in a single location.


A tar czvf archive_name.tar.gz * will include dotfiles in directories below the current working directory. This is an undocumented GNU tar "feature".


This is a symmetric block cipher, used to encrypt files on a single system or local network, as opposed to the "public key" cipher class, of which pgp is a well-known example.


Creates a temporary directory when invoked with the -d option.