| Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting | ||
|---|---|---|
| Prev | Chapter 7. Tests | Next |
file exists
file is a regular file (not a directory or device file)
file is not zero size
file is a directory
file is a block device (floppy, cdrom, etc.)
file is a character device (keyboard, modem, sound card, etc.)
file is a pipe
file is a symbolic link
file is a symbolic link
file is a socket
file (descriptor) is associated with a terminal device
This test option may be used to check whether the stdin ([ -t 0 ]) or stdout ([ -t 1 ]) in a given script is a terminal.
file has read permission (for the user running the test)
file has write permission (for the user running the test)
file has execute permission (for the user running the test)
set-group-id (sgid) flag set on file or directory
If a directory has the sgid flag set, then a file created within that directory belongs to the group that owns the directory, not necessarily to the group of the user who created the file. This may be useful for a directory shared by a workgroup.
set-user-id (suid) flag set on file
A binary owned by root with set-user-id flag set runs with root privileges, even when an ordinary user invokes it. [1] This is useful for executables (such as pppd and cdrecord) that need to access system hardware. Lacking the suid flag, these binaries could not be invoked by a non-root user.
-rwsr-xr-t 1 root 178236 Oct 2 2000 /usr/sbin/pppd |
sticky bit set
Commonly known as the "sticky bit", the save-text-mode flag is a special type of file permission. If a file has this flag set, that file will be kept in cache memory, for quicker access. [2] If set on a directory, it restricts write permission. Setting the sticky bit adds a t to the permissions on the file or directory listing.
drwxrwxrwt 7 root 1024 May 19 21:26 tmp/ |
you are owner of file
group-id of file same as yours
file modified since it was last read
file f1 is newer than f2
file f1 is older than f2
files f1 and f2 are hard links to the same file
"not" -- reverses the sense of the tests above (returns true if condition absent).
Example 7-4. Testing for broken links
1 #!/bin/bash
2 # broken-link.sh
3 # Written by Lee bigelow <ligelowbee@yahoo.com>
4 # Used with permission.
5
6 #A pure shell script to find dead symlinks and output them quoted
7 #so they can be fed to xargs and dealt with :)
8 #eg. broken-link.sh /somedir /someotherdir|xargs rm
9 #
10 #This, however, is a better method:
11 #
12 #find "somedir" -type l -print0|\
13 #xargs -r0 file|\
14 #grep "broken symbolic"|
15 #sed -e 's/^\|: *broken symbolic.*$/"/g'
16 #
17 #but that wouldn't be pure bash, now would it.
18 #Caution: beware the /proc file system and any circular links!
19 ##############################################################
20
21
22 #If no args are passed to the script set directorys to search
23 #to current directory. Otherwise set the directorys to search
24 #to the agrs passed.
25 ####################
26 [ $# -eq 0 ] && directorys=`pwd` || directorys=$@
27
28 #Setup the function linkchk to check the directory it is passed
29 #for files that are links and don't exist, then print them quoted.
30 #If one of the elements in the directory is a subdirectory then
31 #send that send that subdirectory to the linkcheck function.
32 ##########
33 linkchk () {
34 for element in $1/*; do
35 [ -h "$element" -a ! -e "$element" ] && echo \"$element\"
36 [ -d "$element" ] && linkchk $element
37 # Of course, '-h' tests for symbolic link, '-d' for directory.
38 done
39 }
40
41 #Send each arg that was passed to the script to the linkchk function
42 #if it is a valid directoy. If not, then print the error message
43 #and usage info.
44 ################
45 for directory in $directorys; do
46 if [ -d $directory ]
47 then linkchk $directory
48 else
49 echo "$directory is not a directory"
50 echo "Usage: $0 dir1 dir2 ..."
51 fi
52 done
53
54 exit 0 |
Example 29-1, Example 10-7, Example 10-3, Example 29-3, and Example A-2 also illustrate uses of the file test operators.
| [1] | Be aware that suid binaries may open security holes and that the suid flag has no effect on shell scripts. |
| [2] | On modern UNIX systems, the sticky bit is no longer used for files, only on directories. |