Conditions in bash scripting (if statements)

 

Shell中判断语句if中-z至-d的意思 - sunny_2015 - 博客园 https://www.cnblogs.com/coffy/p/5748292.html

 

Conditions in bash scripting (if statements) - Linux Academy Blog https://linuxacademy.com/blog/linux/conditions-in-bash-scripting-if-statements/

 

 

Table of conditions

The following table list the condition possibilities for both the single- and the double-bracket syntax. Save a single exception, the examples are given in single-bracket syntax, but are always compatible with double brackets.

1. File-based conditions:
Condition	True if	Example/explanation
[ -a existingfile ]	file ‘existingfile’ exists.	if [ -a tmp.tmp ]; then rm -f tmp.tmp # Make sure we’re not bothered by an old temporary file fi
[ -b blockspecialfile ]	file ‘blockspecialfile’ exists and is block special.	Block special files are special kernel files found in /dev, mainly used for ATA devices like hard disks, cd-roms and floppy disks.   if [ -b /dev/fd0 ]; then dd if=floppy.img of=/dev/fd0 # Write an image to a floppy fi
[ -c characterspecialfile ]	file ‘characterspecialfile’ exists and is character special.	Character special files are special kernel files found in /dev, used for all kinds of purposes (audio hardware, tty’s, but also /dev/null).   if [ -c /dev/dsp ]; then cat raw.wav > /dev/dsp # This actually works for certain raw wav files fi
[ -d directory ]	file ‘directory’ exists and is a directory.	In UNIX-style, directories are a special kind of file.   if [ -d ~/.kde ]; then echo “You seem to be a kde user.” fi
[ -e existingfile ]	file ‘existingfile’ exists.	(same as -a, see that entry for an example)
[ -f regularfile ]	file ‘regularfile’ exists and is a regular file.	A regular file is neither a block or character special file nor a directory.   if [ -f ~/.bashrc ]; then source ~/.bashrc fi
[ -g sgidfile ]	file ‘sgidfile’ exists and is set-group-ID.	When the SGID-bit is set on a directory, all files created in that directory will inherit the group of the directory.   if [ -g . ]; then echo “Created files are inheriting the group ‘$(ls -ld . | awk ‘{ print $4 }’)’ from the working directory.” fi
[ -G fileownedbyeffectivegroup ]	file ‘fileownedbyeffectivegroup’ exists and is owned by the effective group ID.	The effective group id is the primary group id of the executing user.   if [ ! -G file ]; then # An exclamation mark inverts the outcome of the condition following it chgrp $(id -g) file # Change the group if it’s not the effective one fi
[ -h symboliclink ]	file ‘symboliclink’ exists and is a symbolic link.	if [ -h $pathtofile ]; then pathtofile=$(readlink -e $pathtofile) # Make sure $pathtofile contains the actual file and not a symlink to it fi
[ -k stickyfile ]	file ‘stickyfile’ exists and has its sticky bit set.	The sticky bit has got quite a history, but is now used to prevent world-writable directories from having their contents deletable by anyone.   if [ ! -k /tmp ]; then # An exclamation mark inverts the outcome of the condition following it echo “Warning! Anyone can delete and/or rename your files in /tmp!” fi
[ -L symboliclink ]	file ‘symboliclink’ exists and is a symbolic link.	(same as -h, see that entry for an example)
[ -N modifiedsincelastread ]	file ‘modifiedsincelastread’ exists and was modified after the last read.	if [ -N /etc/crontab ]; then killall -HUP crond # SIGHUP makes crond reread all crontabs fi
[ -O fileownedbyeffectiveuser ]	file ‘fileownedbyeffectiveuser’ exists and is owned by the user executing the script.	if [ -O file ]; then chmod 600 file # Makes the file private, which is a bad idea if you don’t own it fi
[ -p namedpipe ]	file ‘namedpipe’ exists and is a named pipe.	A named pipe is a file in /dev/fd/ that can be read just once. See my bash tutorial for a case in which it’s used.   if [ -p $file ]; then cp $file tmp.tmp # Make sure we’ll be able to read file=”tmp.tmp”    # the file as many times as we like fi
[ -r readablefile ]	file ‘readablefile’ exists and is readable to the script.	if [-r file ]; then content=$(cat file) # Set $content to the content of the file fi
[ -s nonemptyfile ]	file ‘nonemptyfile’ exists and has a size of more than 0 bytes.	if [ -s logfile ]; then gzip logfile    # Backup the old logfile touch logfile # before creating a fresh one. fi
[ -S socket ]	file ‘socket’ exists and is a socket.	A socket file is used for inter-process communication, and features an interface similar to a network connection.   if [ -S /var/lib/mysql/mysql.sock ]; then mysql –socket=/var/lib/mysql/mysql.sock #See this MySQL tip fi
[ -t openterminal ]	file descriptor ‘openterminal’ exists and refers to an open terminal.	Virtually everything is done using files on Linux/UNIX, and the terminal is no exception.   if [ -t /dev/pts/3 ]; then echo -e “nHello there. Message from terminal $(tty) to you.” > /dev/pts/3 #Anyone using that terminal will actually see this message! fi
[ -u suidfile ]	file ‘suidfile’ exists and is set-user-ID.	Setting the suid-bit on a file causes execution of that file to be done with the credentials of the owner of the file, not of the executing user.   if [ -u executable ]; then echo “Running program executable as user $(ls -l executable | awk ‘{ print $3 }’).” fi
[ -w writeablefile ]	file ‘writeablefile’ exists and is writeable to the script.	if [ -w /dev/hda ]; then grub-install /dev/hda fi
[ -x executablefile ]	file ‘executablefile’ exists and is executable for the script.	Note that the execute permission on a directory means that it’s searchable (you can see which files it contains).   if [ -x /root ]; then echo “You can view the contents of the /root directory.” fi
[ newerfile -nt olderfile ]	file ‘newerfile’ was changed more recently than ‘olderfile’, or if ‘newerfile’ exists and ‘olderfile’ doesn’t.	if [ story.txt1 -nt story.txt ]; then echo “story.txt1 is newer than story.txt; I suggest continuing with the former.” fi
[ olderfile -ot newerfile ]	file ‘olderfile’ was changed longer ago than ‘newerfile’, or if ‘newerfile’ exists and ‘olderfile’ doesn’t.	if [ /mnt/remote/remotefile -ot localfile ]; then cp -f localfile /mnt/remote/remotefile # Make sure the remote location has the newest version of the file, too fi
[ same -ef file ]	file ‘same’ and file ‘file’ refer to the same device/inode number.	if [ /dev/cdrom -ef /dev/dvd ]; then echo “Your primary cd drive appears to read dvd’s, too.” fi
2. String-based conditions:
Condition	True if	Example/explanation
[ STRING1 == STRING2 ]	STRING1 is equal to STRING2.	if [ “$1” == “moo” ]; then echo $cow # Ever tried executing ‘apt-get moo’? fiNote: you can also use a single “=” instead of a double one.
[ STRING1 != STRING2 ]	STRING1 is not equal to STRING2.	if [ “$userinput” != “$password” ]; then echo “Access denied! Wrong password!” exit 1 # Stops script execution right here fi
[ STRING1 > STRING2 ]	STRING1 sorts after STRING2 in the current locale (lexographically).	The backslash before the angle bracket is there because the bracket needs to be escaped to be interpreted correctly. As an example we have a basic bubble sort:   (Don’t feel ashamed if you don’t understand this, it is a more complex example) array=( linux tutorial blog ) swaps=1 while (( swaps > 0 )); do swaps=0 for (( i=0; i < (( ${#array[@]} – 1 )) ; i++ )); do if [ “${array[$i]}” > “${array[$(( i + 1 ))]}” ]; then # Here is the sorting condition tempstring=${array[$i]} array[$i]=${array[$(( i + 1 ))]} array[$(( i + 1 ))]=$tempstring (( swaps=swaps + 1 )) fi done done echo ${array[@]} # Returns “blog linux tutorial”
[ STRING1 < STRING2 ]	STRING1 sorts before STRING2 in the current locale (lexographically).
[ -n NONEMPTYSTRING ]	NONEMPTYSTRING has a length of more than zero.	This condition only accepts valid strings, so be sure to quote anything you give to it.   if [ -n “$userinput” ]; then userinput=parse($userinput) # Only parse if the user actually gave some input. fi Note that you can also omit the “-n”, as brackets with just a string in it behave the same.
[ -z EMPTYSTRING ]	EMPTYSTRING is an empty string.	This condition also accepts non-string input, like an uninitialized variable:   if [ -z $uninitializedvar ]; then uninitializedvar=”initialized” # -z returns true on an uninitialized variable, so we initialize it here. fi
Double-bracket syntax only: [[ STRING1 =~ REGEXPATTERN ]]	STRING1 matches REGEXPATTERN.	If you are familiar with Regular Expressions, you can use this conditions to perform a regex match.   if [[ “$email” =~ “b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+.[A-Za-z]{2,4}b” ]]; then echo “$email contains a valid e-mail address.” fi
3. Arithmetic (number-based) conditions:
Condition	True if	Example/explanation
[ NUM1 -eq NUM2 ]	NUM1 is EQual to NUM2.	These conditions only accept integer numbers. Strings will be converted to integer numbers, if possible. Some random examples:   if [ $? -eq 0 ]; then # $? returns the exit status of the previous command echo “Previous command ran succesfully.” fi if [ $(ps -p $pid -o ni=) -ne $(nice) ]; then echo “Process $pid is running with a non-default nice value” fi if [ $num -lt 0 ]; then echo “Negative numbers not allowed; exiting…” exit 1 fi
[ NUM1 -ne NUM2 ]	NUM1 is Not Equal to NUM2.
[ NUM1 -gt NUM2 ]	NUM1 is Greater Than NUM2.
[ NUM1 -ge NUM2 ]	NUM1 is Greater than orEqual to NUM2.
[ NUM1 -lt NUM2 ]	NUM1 is Less Than NUM2.
[ NUM1 -le NUM2 ]	NUM1 is Less than or Equal to NUM2.
4. Miscellaneous conditions:
Condition	True if	Example/explanation
[ -o shelloption ]	shell option ‘shelloption’ is enabled.	Shell options modify the behaviour of bash, except a few unmodifiable ones that indicate the shell status.   if [ ! -o checkwinsize ] # An exclamation mark inverts the outcome of the condition following it echo “Shell option checkwinsize is disabled; enabling it so you can resize you terminal window without problems.” shopt -s checkwinsize # This shell option is modifiable fi if [ -o login_shell ]; then echo “This a a login shell.” # This shell option is not modifiable fi

With the double-parenthesis syntax, you can use the following conditions:

5. Double-parenthesis syntax conditions:
Condition	True if	Example/explanation
(( NUM1 == NUM2 ))	NUM1 is equal to NUM2.	These conditions only accept integer numbers. Strings will be converted to integer numbers, if possible. Some random examples:   if (( $? == 0 )); then # $? returns the exit status of the previous command echo “Previous command ran succesfully.” fi if (( $(ps -p $pid -o ni=) != $(nice) )); then echo “Process $pid is running with a non-default nice value” fi if (( $num < 0 )); then echo “Negative numbers not allowed; exiting…” exit 1 fi
(( NUM1 != NUM2 ))	NUM1 is not equal to NUM2.
(( NUM1 > NUM2 ))	NUM1 is greater than NUM2.
(( NUM1 >= NUM2 ))	NUM1 is greater than or equal to NUM2.
(( NUM1 < NUM2 ))	NUM1 is less than NUM2.
(( NUM1 <= NUM2 ))	NUM1 is less than or equal to NUM2.

After this dry information load, here’s a bit of explanation for those who want to know more…