Chapter 15. 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.

15.1. 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 useful options are -S, sort listing by file size, -t, sort by file modification time, -b, show escape characters, and -i, show file inodes (see Example 15-4).

Example 15-1. 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 DEVICE=cdrom
  10 # DEVICE="0,0"     For older versions of cdrecord
  11 DEFAULTDIR=/opt  # This is the directory containing the data to be burned.
  12                  # Make sure it exists.
  13                  # Exercise: Add a test for this.
  15 # Uses Joerg Schilling's "cdrecord" package:
  16 #
  18 #  If this script invoked as an ordinary user, may need to suid cdrecord
  19 #+ chmod u+s /usr/bin/cdrecord, as root.
  20 #  Of course, this creates a security hole, though a relatively minor one.
  22 if [ -z "$1" ]
  23 then
  25   # Default directory, if not specified on command line.
  26 else
  28 fi
  30 # Create a "table of contents" file.
  32 # The "l" option gives a "long" file listing.
  33 # The "R" option makes the listing recursive.
  34 # The "F" option marks the file types (directories get a trailing /).
  35 echo "Creating table of contents."
  37 # Create an image file preparatory to burning it onto the CDR.
  38 mkisofs -r -o $IMAGEFILE $IMAGE_DIRECTORY
  39 echo "Creating ISO9660 file system image ($IMAGEFILE)."
  41 # Burn the CDR.
  42 echo "Burning the disk."
  43 echo "Please be patient, this will take a while."
  44 cdrecord -v -isosize speed=$SPEED dev=$DEVICE $IMAGEFILE
  46 exit $?

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 # Uses of 'cat'
   2 cat filename                          # Lists the file.
   4 cat file.1 file.2 file.3 > file.123   # Combines three files into one.
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 15-25 and Example 15-21.


In a pipe, it may be more efficient to redirect the stdin to a file, rather than to cat the file.

   1 cat filename | tr a-z A-Z
   3 tr a-z A-Z < filename   #  Same effect, but starts one less process,
   4                         #+ and also dispenses with the pipe.

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


reverses each line of a file, and outputs to stdout. This does not have the same effect as tac, as it preserves the order of the lines, but flips each one around (mirror image).

 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 15-6).


Particularly useful are the -a archive flag (for copying an entire directory tree), the -u update flag (which prevents overwriting identically-named newer files), and the -r and -R recursive flags.

   1 cp -u source_dir/* dest_dir
   2 #  "Synchronize" dest_dir to source_dir
   3 #+  by copying over all newer and not previously existing files.


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 9-19 and Example A-2.


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.


The rm command will, by itself, fail to remove filenames beginning with a dash.

 bash$ rm -badname
 rm: invalid option -- b
 Try `rm --help' for more information.

One way to accomplish this is to preface the filename to be removed with a dot-slash .
 bash$ rm ./-badname
Another method is to precede the filename with a " -- ".
 bash$ rm -- -badname


When used with the recursive flag -r, this command removes files all the way down the directory tree from the current directory. A careless rm -rf * can wipe out a big chunk of a directory structure.


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. For example, 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 or directory (see Example 14-12).

   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 others
   3 # (octal mode).
   5 chmod 444 filename
   6 #  Makes "filename" read-only for all.
   7 #  Modifying the file (for example, with a text editor)
   8 #+ not allowed for a user who does not own the file (except for root),
   9 #+ and even the file owner must force a file-save
  10 #+ if she modifies the file.
  11 #  Same restrictions apply for deleting the file.

   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.
   8 chmod 111 directory-name
   9 #  Gives everyone execute-only permission in a directory.
  10 #  This means that you can execute and READ the files in that directory
  11 #+ (execute permission necessarily includes read permission
  12 #+ because you can't execute a file without being able to read it).
  13 #  But you can't list the files or search for them with the "find" command.
  14 #  These restrictions do not apply to root.
  16 chmod 000 directory-name
  17 #  No permissions at all for that directory.
  18 #  Can't read, write, or execute files in it.
  19 #  Can't even list files in it or "cd" to it.
  20 #  But, you can rename (mv) the directory
  21 #+ or delete it (rmdir) if it is empty.
  22 #  You can even symlink to files in the directory,
  23 #+ but you can't read, write, or execute the symlinks.
  24 #  These restrictions do not apply to root.


Change file attributes. This is analogous to chmod above, but with different options and a different invocation syntax, and it works only on an ext2 filesystem.

One particularly interesting chattr option is i. A chattr +i filename marks the file as immutable. The file cannot be modified, linked to, or deleted, not even by root. This file attribute can be set or removed only by root. In a similar fashion, the a option marks the file as append only.

 root# chattr +i file1.txt
 root# rm file1.txt
 rm: remove write-protected regular file `file1.txt'? y
 rm: cannot remove `file1.txt': Operation not permitted

If a file has the s (secure) attribute set, then when it is deleted its block is zeroed out on the disk.

If a file has the u (undelete) attribute set, then when it is deleted, its contents can still be retrieved (undeleted).

If a file has the c (compress) attribute set, then it will automatically be compressed on writes to disk, and uncompressed on reads.


The file attributes set with chattr do not show in a file listing (ls -l).


Creates links to pre-existings files. A "link" is a reference to a file, an alternate name for it. The ln command permits referencing the linked file by more than one name and is a superior alternative to aliasing (see Example 4-6).

The ln creates only a reference, a pointer to the file only a few bytes in size.

The ln command is most often used with the -s, symbolic or "soft" link flag. Advantages of using the -s flag are that it permits linking across file systems or to directories.

The syntax of the command is a bit tricky. For example: ln -s oldfile newfile links the previously existing oldfile to the newly created link, newfile.


If a file named newfile has previously existed, an error message will result.

Links give the ability to invoke a script (or any other type of executable) with multiple names, and having that script behave according to how it was invoked.

Example 15-2. Hello or Good-bye

   1 #!/bin/bash
   2 # Saying "hello" or "goodbye"
   3 #+          depending on how script is invoked.
   5 # Make a link in current working directory ($PWD) to this script:
   6 #    ln -s goodbye
   7 # Now, try invoking this script both ways:
   8 # ./
   9 # ./goodbye
  12 HELLO_CALL=65
  15 if [ $0 = "./goodbye" ]
  16 then
  17   echo "Good-bye!"
  18   # Some other goodbye-type commands, as appropriate.
  19   exit $GOODBYE_CALL
  20 fi
  22 echo "Hello!"
  23 # Some other hello-type commands, as appropriate.
  24 exit $HELLO_CALL

man, info

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



Dotfiles are files whose names begin with a dot, such as ~/.Xdefaults. Such filenames do not appear in a normal ls listing (although an ls -a will show them), 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.