Chapter 12. External Filters, Programs and Commands

Standard UNIX commands make shell scripts more versatile. The power of scripts comes from coupling system commands and shell directives with simple programming constructs.

12.1. Basic Commands

The first commands a novice learns


The basic file "list" command. It is all too easy to underestimate the power of this humble command. For example, using the -R, recursive option, ls provides a tree-like listing of a directory structure. Other interesting options are -S, sort listing by file size, -t, sort by file modification time, and -i, show file inodes (see Example 12-3).

Example 12-1. Using ls to create a table of contents for burning a CDR disk

   1 #!/bin/bash
   2 #
   3 # Script to automate burning a CDR.
   6 SPEED=2          # May use higher speed if your hardware supports it.
   7 IMAGEFILE=cdimage.iso
   8 CONTENTSFILE=contents
   9 DEFAULTDIR=/opt  # This is the directory containing the data to be burned.
  10                  # Make sure it exists.
  12 # Uses Joerg Schilling's "cdrecord" package.
  13 # (
  15 #  If this script invoked as an ordinary user, need to suid cdrecord
  16 #+ (chmod u+s /usr/bin/cdrecord, as root).
  18 if [ -z "$1" ]
  19 then
  21   # Default directory, if not specified on command line.
  22 else
  24 fi
  26 # Create a "table of contents" file.
  28 # The "l" option gives a "long" file listing.
  29 # The "R" option makes the listing recursive.
  30 # The "F" option marks the file types (directories get a trailing /).
  31 echo "Creating table of contents."
  33 # Create an image file preparatory to burning it onto the CDR.
  34 mkisofs -r -o $IMAGFILE $IMAGE_DIRECTORY
  35 echo "Creating ISO9660 file system image ($IMAGEFILE)."
  37 # Burn the CDR.
  38 cdrecord -v -isosize speed=$SPEED dev=0,0 $IMAGEFILE
  39 echo "Burning the disk."
  40 echo "Please be patient, this will take a while."
  42 exit 0

cat, tac

cat, an acronym for concatenate, lists a file to stdout. When combined with redirection (> or >>), it is commonly used to concatenate files.
   1 cat filename cat file.1 file.2 file.3 > file.123
The -n option to cat inserts consecutive numbers before all lines of the target file(s). The -b option numbers only the non-blank lines. The -v option echoes nonprintable characters, using ^ notation. The -s option squeezes multiple consecutive blank lines into a single blank line.

See also Example 12-21 and Example 12-17.

tac, is the inverse of cat, listing a file backwards from its end.


reverses each line of a file, and outputs to stdout. This is not the same effect as tac, as it preserves the order of the lines, but flips each one around.

 bash$ cat file1.txt
 This is line 1.
 This is line 2.
 bash$ tac file1.txt
 This is line 2.
 This is line 1.
 bash$ rev file1.txt
 .1 enil si sihT
 .2 enil si sihT


This is the file copy command. cp file1 file2 copies file1 to file2, overwriting file2 if it already exists (see Example 12-5).


Particularly useful are the -a archive flag (for copying an entire directory tree) and the -r and -R recursive flags.


This is the file move command. It is equivalent to a combination of cp and rm. It may be used to move multiple files to a directory, or even to rename a directory. For some examples of using mv in a script, see Example 9-17 and Example A-3.


When used in a non-interactive script, mv takes the -f (force) option to bypass user input.

When a directory is moved to a preexisting directory, it becomes a subdirectory of the destination directory.

 bash$ mv source_directory target_directory
 bash$ ls -lF target_directory
 total 1
 drwxrwxr-x    2 bozo  bozo      1024 May 28 19:20 source_directory/


Delete (remove) a file or files. The -f option forces removal of even readonly files, and is useful for bypassing user input in a script.


When used with the recursive flag -r, this command removes files all the way down the directory tree.


Remove directory. The directory must be empty of all files, including invisible "dotfiles", [1] for this command to succeed.


Make directory, creates a new directory. mkdir -p project/programs/December creates the named directory. The -p option automatically creates any necessary parent directories.


Changes the attributes of an existing file (see Example 11-10).

   1 chmod +x filename
   2 # Makes "filename" executable for all users.
   4 chmod u+s filename
   5 # Sets "suid" bit on "filename" permissions.
   6 # An ordinary user may execute "filename" with same privileges as the file's owner.
   7 # (This does not apply to shell scripts.)

   1 chmod 644 filename
   2 # Makes "filename" readable/writable to owner, readable to
   3 # others
   4 # (octal mode).

   1 chmod 1777 directory-name
   2 # Gives everyone read, write, and execute permission in directory,
   3 # however also sets the "sticky bit".
   4 # This means that only the owner of the directory,
   5 # owner of the file, and, of course, root
   6 # can delete any particular file in that directory.


Change file attributes. This has the same effect as chmod above, but with a different invocation syntax, and it works only on an ext2 filesystem.


Creates links to pre-existings files. Most often used with the -s, symbolic or "soft" link flag. This permits referencing the linked file by more than one name and is a superior alternative to aliasing (see Example 4-6).

ln -s oldfile newfile links the previously existing oldfile to the newly created link, newfile.

man, info

These commands access the manual and information pages on system commands and installed utilities. When available, the info pages usually contain a more detailed description than do the man pages.



These are files whose names begin with a dot, such as ~/.Xdefaults. Such filenames do not show up in a normal ls listing, and they cannot be deleted by an accidental rm -rf *. Dotfiles are generally used as setup and configuration files in a user's home directory.