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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 useful options are
-S, sort listing by file size,
-t, sort by file modification time, and
-i, show file inodes (see Example 12-4).
Example 12-1. Using ls to create a table of contents
for burning a CDR disk
# ex40.sh (burn-cd.sh)
# Script to automate burning a CDR.
SPEED=2 # May use higher speed if your hardware supports it.
# DEVICE="0,0" For older versions of cdrecord
DEFAULTDIR=/opt # This is the directory containing the data to be burned.
# Make sure it exists.
# Exercise: Add a test for this.
# Uses Joerg Schilling's "cdrecord" package:
# If this script invoked as an ordinary user, may need to suid cdrecord
#+ chmod u+s /usr/bin/cdrecord, as root.
# Of course, this creates a security hole, though a relatively minor one.
if [ -z "$1" ]
# Default directory, if not specified on command line.
# Create a "table of contents" file.
ls -lRF $IMAGE_DIRECTORY > $IMAGE_DIRECTORY/$CONTENTSFILE
# The "l" option gives a "long" file listing.
# The "R" option makes the listing recursive.
# The "F" option marks the file types (directories get a trailing /).
echo "Creating table of contents."
# Create an image file preparatory to burning it onto the CDR.
mkisofs -r -o $IMAGEFILE $IMAGE_DIRECTORY
echo "Creating ISO9660 file system image ($IMAGEFILE)."
# Burn the CDR.
echo "Burning the disk."
echo "Please be patient, this will take a while."
cdrecord -v -isosize speed=$SPEED dev=$DEVICE $IMAGEFILE
- cat, tac
cat, an acronym for
lists a file to stdout. When
combined with redirection (> or
>>), it is commonly used to concatenate
# Uses of 'cat'
cat filename # Lists the file.
cat file.1 file.2 file.3 > file.123 # Combines three files into one.
The -n option to cat
inserts consecutive numbers before all lines of the
target file(s). The -b option numbers
only the non-blank lines. The -v option
echoes nonprintable characters, using ^
notation. The -s option squeezes multiple
consecutive blank lines into a single blank line.
See also Example 12-25 and Example 12-21.
In a pipe, it may be
more efficient to redirect
the stdin to a file, rather than to
cat the file.
cat filename | tr a-z A-Z
tr a-z A-Z < filename # Same effect, but starts one less process,
#+ and also dispenses with the pipe.
tac, is the inverse of
cat, listing a file backwards from its end.
reverses each line of a file, and outputs to
stdout. This does not have the same effect
as tac, as it preserves the order of
the lines, but flips each one around.
bash$ 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 12-6).
Particularly useful are the -a
archive flag (for copying an entire directory tree)
and the -r and -R
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 9-18 and Example A-2.
When used in a non-interactive script,
mv takes the -f
(force) option to bypass user
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
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
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"
-- 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 (see Example 11-12).
chmod +x filename
# Makes "filename" executable for all users.
chmod u+s filename
# Sets "suid" bit on "filename" permissions.
# An ordinary user may execute "filename" with same privileges as the file's owner.
# (This does not apply to shell scripts.)
chmod 644 filename
# Makes "filename" readable/writable to owner, readable to
# (octal mode).
chmod 1777 directory-name
# Gives everyone read, write, and execute permission in directory,
# however also sets the "sticky bit".
# This means that only the owner of the directory,
# owner of the file, and, of course, root
# can delete any particular file in that directory.
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
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. An advantage of using the
-s flag is that it permits linking across
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, it will be deleted when the filename
newfile is preempted as the name for a
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 12-2. Hello or Good-bye
# hello.sh: Saying "hello" or "goodbye"
#+ depending on how script is invoked.
# Make a link in current working directory ($PWD) to this script:
# ln -s hello.sh goodbye
# Now, try invoking this script both ways:
if [ $0 = "./goodbye" ]
# Some other goodbye-type commands, as appropriate.
# Some other hello-type commands, as appropriate.
- 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.
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