Bash File Test Operators
We often need to check files or folders in a bash script. Following are file test operators in bash.\
-a¶
Returns true if file exists.
Note
It is identical in effect to -e, but it is deprecated and its use is discouraged.
-b¶
Returns true if file is a block device.
Note
A block device (e.g., hard drives, CDROM drives, flash drives) reads and/or writes data in chunks, or blocks, in contrast to a character device (e.g., keyboards, modems, sound cards), which acesses data in character units.
device="/dev/sda2"
if [ -b "$device" ]
then
echo "$device is a block device."
fi
# /dev/sda2 is a block device.
-c¶
Returns true if file is a character device.
device1="/dev/ttyS1" # PCMCIA modem card.
if [ -c "$device1" ]
then
echo "$device1 is a character device."
fi
# /dev/ttyS1 is a character device.
-d¶
Returns true if file is a directory.
-e¶
Returns true if file exists.
-f¶
Returns true if file is a regular file, i.e., not a device file or a directory.
-g¶
Returns true if 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.
-G¶
Returns true if group-id of the file is same as you.
-h¶
Returns true if file is a symbolic link.
-L¶
Returns true if file is a symbolic link.
-N¶
Returns true if file is modified since last read.
-O¶
Returns true if you are the owner of the file.
-p¶
Returns true if file is a pipe.
Note
Passes the output (stdout) of a previous command to the input (stdin) of the next one, or to the shell. This is a method of chaining commands together.
function show_input_type()
{
[ -p /dev/fd/0 ] && echo PIPE || echo STDIN
}
show_input_type "Input" # STDIN
echo "Input" | show_input_type # PIPE
# This example courtesy of Carl Anderson.
-s¶
Returns true if file is not zero size.
-S¶
Returns true if file is a socket.
Note
A socket is a communications node associated with a specific I/O port. (This is analogous to a hardware socket, or receptacle, for a connecting cable.) It permits data transfer between hardware devices on the same machine, between machines on the same network, between machines across different networks, and, of course, between machines at different locations on the Internet.
-t¶
Returns true if file 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.
-u¶
Returns true if set-user-id (suid) flag set on file
Note
A binary owned by root with set-user-id flag set runs with root privileges, even when an ordinary user invokes it. 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.
-w¶
Returns true if file has write permission (for the user running the test).
-x¶
Returns true if file has execute permission (for the user running the test).
f1 -nt f2¶
Returns true if file f1 is newer than f2.
f1 -ot f2¶
Returns true if file f1 is older than f2.
f1 -ef f2¶
Returns true if 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¶
#!/bin/bash
# fileinfo.sh
FILES="/usr/sbin/accept
/usr/sbin/pwck
/usr/sbin/chroot
/usr/bin/fakefile
/sbin/badblocks
/sbin/ypbind" # List of files you are curious about.
# Threw in a dummy file, /usr/bin/fakefile.
echo
for file in $FILES
do
if [ ! -e "$file" ] # Check if file exists.
then
echo "$file does not exist."; echo
continue # On to next.
fi