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 |
[ -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 |
[ -d directory ] | file ‘directory’ exists and is a directory. | In UNIX-style, directories are a special kind of file.
if [ -d ~/.kde ]; then |
[ -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 |
[ -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 |
[ -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 |
[ -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 |
[ -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 $file ]; then |
[ -r readablefile ] | file ‘readablefile’ exists and is readable to the script. | if [-r file ]; then content=$(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 |
[ -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 |
[ -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 |
[ -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 |
[ 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” != “$password” ]; then echo “Access denied! Wrong password!” exit 1 # Stops script execution right here fi |
[ STRING1 > STRING2 ] | STRING1 sorts after STRING2 in the current locale (lexographically). | The backslash before the angle bracket is there because the bracket needs to be escaped to be interpreted correctly. As an example we have a basic bubble sort:
(Don’t feel ashamed if you don’t understand this, it is a more complex example) array=( linux tutorial blog ) swaps=0 |
[ STRING1 < STRING2 ] | STRING1 sorts before STRING2 in the current locale (lexographically). | |
[ -n NONEMPTYSTRING ] | NONEMPTYSTRING has a length of more than zero. | This condition only accepts valid strings, so be sure to quote anything you give to it.
if [ -n “$userinput” ]; then 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 $uninitializedvar ]; then |
Double-bracket syntax only: [[ STRING1 =~ REGEXPATTERN ]] |
STRING1 matches REGEXPATTERN. | If you are familiar with Regular Expressions, you can use this conditions to perform a regex match.
if [[ “$email” =~ “b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+.[A-Za-z]{2,4}b” ]]; then |
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 if [ $(ps -p $pid -o ni=) -ne $(nice) ]; then if [ $num -lt 0 ]; then |
[ 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 if [ -o login_shell ]; then |
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 if (( $(ps -p $pid -o ni=) != $(nice) )); then if (( $num < 0 )); then |
(( 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…
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