Basic External Commands

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, -v, sort by (numerical) version numbers embedded in the filenames, -b, show escape characters, and -i, show file inodes (see TODO Example 16-4). [1]

bash$ ls -l
-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter10.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter11.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter12.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter1.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter2.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter3.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:49 Chapter_headings.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:49 Preface.txt
bash$ ls -lv
 total 0
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:49 Chapter_headings.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:49 Preface.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter1.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter2.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter3.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter10.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter11.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter12.txt

Tip: The ls command returns a non-zero exit status when attempting to list a non-existent file.

bash$ ls abc
ls: abc: No such file or directory
bash$ echo $?
2

Using ls to create a table of contents for burning a CDR disk

#!/bin/bash
# ex40.sh (burn-cd.sh)
# Script to automate burning a CDR.
SPEED=10         # May use higher speed if your hardware supports it.
IMAGEFILE=cdimage.iso
CONTENTSFILE=contents
# DEVICE=/dev/cdrom     For older versions of cdrecord
DEVICE="1,0,0"
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:
# http://www.fokus.fhg.de/usr/schilling/cdrecord.html
#  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 "$1" ]
then
  IMAGE_DIRECTORY=$DEFAULTDIR
  # Default directory, if not specified on command-line.
else
    IMAGE_DIRECTORY=$1
fi
# 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."
wodim -v -isosize dev=$DEVICE $IMAGEFILE
#  In newer Linux distros, the "wodim" utility assumes the
#+ functionality of "cdrecord."
exitcode=$?
echo "Exit code = $exitcode"
exit $exitcode

cat, an acronym for concatenate, lists a file to stdout. When combined with redirection (> or >>), it is commonly used to concatenate files.

# 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 TODO Example 16-28 and Example 16-24.

Note: 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 (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 TODO Example 16-6).

Tip: 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.

cp -u source_dir/* dest_dir
#  "Synchronize" dest_dir to source_dir
#+  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 TODO Example 10-11 and Example A-2.

Note: 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.

Note: The rm command will, by itself, fail to remove filenames beginning with a dash. Why? Because rm sees a dash-prefixed filename as an option.

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

One clever workaround is to precede the filename with a -- (the end-of-options flag).

bash$ rm -- -badname

Another method to is to preface the filename to be removed with a dot-slash .

bash$ rm ./-badname

Warning: 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.

Changes the attributes of an existing file or directory (see TODO Example 15-14).

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 others
#+ (octal mode).
chmod 444 filename
#  Makes "filename" read-only for all.
#  Modifying the file (for example, with a text editor)
#+ not allowed for a user who does not own the file (except for root),
#+ and even the file owner must force a file-save
#+ if she modifies the file.
#  Same restrictions apply for deleting the file.
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.
chmod 111 directory-name
#  Gives everyone execute-only permission in a directory.
#  This means that you can execute and READ the files in that directory
#+ (execute permission necessarily includes read permission
#+ because you can't execute a file without being able to read it).
#  But you can't list the files or search for them with the "find" command.
#  These restrictions do not apply to root.
chmod 000 directory-name
#  No permissions at all for that directory.
#  Can't read, write, or execute files in it.
#  Can't even list files in it or "cd" to it.
#  But, you can rename (mv) the directory
#+ or delete it (rmdir) if it is empty.
#  You can even symlink to files in the directory,
#+ but you can't read, write, or execute the symlinks.
#  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 ext2/ext3 filesystems.

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 overwritten with binary zeroes. [3]

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.

Note: 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 TODO 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.

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

Which type of link to use?

Both of these [types of links] provide a certain measure of dual reference -- if you edit the contents of the file using any name, your changes will affect both the original name and either a hard or soft new name. The differences between them occurs when you work at a higher level. The advantage of a hard link is that the new name is totally independent of the old name -- if you remove or rename the old name, that does not affect the hard link, which continues to point to the data while it would leave a soft link hanging pointing to the old name which is no longer there. The advantage of a soft link is that it can refer to a different file system (since it is just a reference to a file name, not to actual data). And, unlike a hard link, a symbolic link can refer to a directory.

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.

#!/bin/bash
# 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:
# ./hello.sh
# ./goodbye
HELLO_CALL=65
GOODBYE_CALL=66
if [ $0 = "./goodbye" ]
then
  echo "Good-bye!"
  # Some other goodbye-type commands, as appropriate.
  exit $GOODBYE_CALL
fi
echo "Hello!"
# Some other hello-type commands, as appropriate.
exit $HELLO_CALL

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.

There have been various attempts at "automating" the writing of man pages. For a script that makes a tentative first step in that direction, see TODO Example A-39.