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7.2. File test operators
- -e
file exists
- -a
file exists
This is identical in effect to -e. It has been "deprecated," and its use is discouraged.
- -f
file is a regular file (not a directory or device file)
- -s
file is not zero size
- -d
file is a directory
- -b
file is a block device (floppy, cdrom, etc.)
- -c
file is a character device (keyboard, modem, sound card, etc.)
- -p
file is a pipe
- -h
file is a symbolic link
- -L
file is a symbolic link
- -S
file is a socket
- -t
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.
- -r
file has read permission (for the user running the test)
- -w
file has write permission (for the user running the test)
- -x
file has execute permission (for the user running the test)
- -g
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.
- -u
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.
A file with the suid flag set shows an s in its permissions.-rwsr-xr-t 1 root 178236 Oct 2 2000 /usr/sbin/pppd
- -k
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.
If a user does not own a directory that has the sticky bit set, but has write permission in that directory, he can only delete files in it that he owns. This keeps users from inadvertently overwriting or deleting each other's files in a publicly accessible directory, such as /tmp.drwxrwxrwt 7 root 1024 May 19 21:26 tmp/
- -O
you are owner of file
- -G
group-id of file same as yours
- -N
file modified since it was last read
- f1 -nt f2
file f1 is newer than f2
- f1 -ot f2
file f1 is older than f2
- f1 -ef 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
#!/bin/bash
# broken-link.sh
# Written by Lee bigelow <ligelowbee@yahoo.com>
# Used with permission.
#A pure shell script to find dead symlinks and output them quoted
#so they can be fed to xargs and dealt with :)
#eg. broken-link.sh /somedir /someotherdir|xargs rm
#
#This, however, is a better method:
#
#find "somedir" -type l -print0|\
#xargs -r0 file|\
#grep "broken symbolic"|
#sed -e 's/^\|: *broken symbolic.*$/"/g'
#
#but that wouldn't be pure bash, now would it.
#Caution: beware the /proc file system and any circular links!
##############################################################
#If no args are passed to the script set directorys to search
#to current directory. Otherwise set the directorys to search
#to the agrs passed.
####################
[ $# -eq 0 ] && directorys=`pwd` || directorys=$@
#Setup the function linkchk to check the directory it is passed
#for files that are links and don't exist, then print them quoted.
#If one of the elements in the directory is a subdirectory then
#send that send that subdirectory to the linkcheck function.
##########
linkchk () {
for element in $1/*; do
[ -h "$element" -a ! -e "$element" ] && echo \"$element\"
[ -d "$element" ] && linkchk $element
# Of course, '-h' tests for symbolic link, '-d' for directory.
done
}
#Send each arg that was passed to the script to the linkchk function
#if it is a valid directoy. If not, then print the error message
#and usage info.
################
for directory in $directorys; do
if [ -d $directory ]
then linkchk $directory
else
echo "$directory is not a directory"
echo "Usage: $0 dir1 dir2 ..."
fi
done
exit 0 |
Example 28-1, Example 10-7, Example 10-3, Example 28-3, and Example A-1 also illustrate uses of the file test operators.
Notes
| [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. |
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Generated: 2007-01-26 17:57:30