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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).The ls command returns a non-zero exit status when attempting to list a non-existent file.
ls: abc: No such file or directory
Example 15-1. 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 # 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: # 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." cdrecord -v -isosize speed=$SPEED dev=$DEVICE $IMAGEFILE # In newer Linux distros, the "wodim" utility may assume the #+ functionality of "cdrecord." 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.
# Uses of 'cat' cat filename # Lists the file. cat file.1 file.2 file.3 > file.123 # Combines three files into one.
-noption to cat inserts consecutive numbers before all lines of the target file(s). The
-boption numbers only the non-blank lines. The
-voption echoes nonprintable characters, using ^ notation. The
-soption squeezes multiple consecutive blank lines into a single blank line.
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).
This is line 1. This is line 2.
This is line 2. This is line 1.
.1 enil si sihT .2 enil si sihT
This is the file copy command.
cp file1 file2copies
file2if it already exists (see Example 15-6).
Particularly useful are the
-aarchive flag (for copying an entire directory tree), the
-uupdate flag (which prevents overwriting identically-named newer files), and the
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 Example 9-20 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.
mv source_directory target_directory
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
-foption 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. Why? Because rm sees a dash-prefixed filename as an option.
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).
rm -- -badnameAnother method to is to preface the filename to be removed with a
Remove directory. The directory must be empty of all files -- including "invisible" dotfiles  -- for this command to succeed.
Make directory, creates a new directory. For example,
mkdir -p project/programs/Decembercreates the named directory. The
-poption automatically creates any necessary parent directories.
Changes the attributes of an existing file or directory (see Example 14-13).
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
filenamemarks 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
aoption marks the file as append only.
chattr +i 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. 
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
-sflag 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 newfilelinks the previously existing
oldfileto the newly created link,
If a file named
newfilehas 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
#!/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
- 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.
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 Example A-41.
Dotfiles are files whose
names begin with a dot, such as
This particular feature may not yet be implemented in the version of the ext2/ext3 filesystem installed on your system. Check the documentation for your Linux distro.