Linux bash script and or

There are a few cases in which you should notquote, but they are rare. You will see one of them further on in the tutorial.

Also, there are two things that may be useful to know:

You can invert a condition by putting an “!” in front of it. Example:

Be sure to place the “!” inside the brackets!

You can combine conditions by using certain operators. For the single-bracket syntax that we’ve been using so far, you can use “-a” for and and “-o” for or. Example:

The above condition will return true if $foo contains an integer greater than or equal to 3 and Less Than 10. You can read more about these combining expressions at the respective condition syntaxes.

And, one more basic thing: don’t forget that conditions can also be used in other statements, like while and until. It is outside the scope of this tutorial to explain those, but you can read about them at the Bash Guide for Beginners.Anyway, I’ve only shown you conditions between single brackets so far. There are more syntaxes, however, as you will read in the next section.

Different condition syntaxes

Bash features different syntaxes for conditions. I will list the three of them:

1. Single-bracket syntax

This is the condition syntax you have already seen in the previous paragraphs; it’s the oldest supported syntax. It supports three types of conditions:

File-based conditions
Allows different kinds of checks on a file. Example:

The above condition is true if the file ‘symboliclink’ exists and is a symbolic link. For more file-based conditions see the table below.

String-based conditions

Allows checks on a string and comparing of strings. Example one:

The above condition is true if $emptystring is an empty string or an uninitialized variable. Example two:

The above condition is true if $stringvar1 contains just the string “cheese”. For more string-based conditions see the table below.

Arithmetic (number-based) conditions

Allows comparing integer numbers. Example:

The above condition returns true if $num is less than 1. For more arithmetic conditions see the table below.

2. Double-bracket syntax

You may have encountered conditions enclosed in double square brackets already, which look like this:

The double-bracket syntax serves as an enhanced version of the single-bracket syntax; it mainly has the same features, but also some important differences with it. I will list them here:

The first difference can be seen in the above example; when comparing strings, the double-bracket syntax features shell globbing. This means that an asterisk (“*”) will expand to literally anything, just as you probably know from normal command-line usage. Therefore, if $stringvar contains the phrase “string” anywhere, the condition will return true. Other forms of shell globbing are allowed, too. If you’d like to match both “String” and “string”, you could use the following syntax:

Note that only general shell globbing is allowed. Bash-specific things like <1..4>or will not work. Also note that the globbing will not work if you quote the right string. In this case you should leave it unquoted.

The second difference is that word splitting is prevented. Therefore, you could omit placing quotes around string variables and use a condition like the following without problems:

Nevertheless, the quoting string variables remains a good habit, so I recommend just to keep doing it.

The third difference consists of not expanding filenames. I will illustrate this difference using two examples, starting with the old single-bracket situation:

The above condition will return true if there is one single file in the working directory that has a .sh extension. If there are none, it will return false. If there are several .sh files, bash will throw an error and stop executing the script. This is because *.sh is expanded to the files in the working directory. Using double brackets prevents this:

The above condition will return true only if there is a file in the working directory called “*.sh”, no matter what other .sh files exist. The asterisk is taken literally, because the double-bracket syntax does not expand filenames.

The fourth difference is the addition of more generally known combining expressions, or, more specific, the operators “&&” and “||”. Example:

The above condition returns true if $num is equal to 3 and $stringvar is equal to “foo”. The -a and -o known from the single-bracket syntax is supported, too.Note that the and operator has precedence over the or operator, meaning that “&&” or “-a” will be evaluated before “||” or “-o”.

The fifth difference is that the double-bracket syntax allows regex pattern matching using the “=

” operator. See the table for more information.

3. Double-parenthesis syntax

There also is another syntax for arithmetic (number-based) conditions, most likely adopted from the Korn shell:

The above condition is true if $num is less than or equal to 5. This syntax may seem more familiar to programmers. It features all the ‘normal’ operators, like “==”, “ =”. It supports the “&&” and “||” combining expressions (but not the -a and -o ones!). It is equivalent to the built-in let command.

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:

/.kde ]; thenecho “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

/.bashrcfi [ -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 . ]; thenecho “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 itchgrp $(id -g) file # Change the group if it’s not the effective onefi [ -h symboliclink ] file ‘symboliclink’ exists and is a symbolic link. if [ -h $pathtofile ]; thenpathtofile=$(readlink -e $pathtofile) # Make sure $pathtofile contains the actual file and not a symlink to itfi [ -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 itecho “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 ]; thenkillall -HUP crond # SIGHUP makes crond reread all crontabsfi [ -O fileownedbyeffectiveuser ] file ‘fileownedbyeffectiveuser’ exists and is owned by the user executing the script. if [ -O file ]; thenchmod 600 file # Makes the file private, which is a bad idea if you don’t own itfi [ -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 ]; thencp $file tmp.tmp # Make sure we’ll be able to readfile=”tmp.tmp” # the file as many times as we likefi [ -r readablefile ] file ‘readablefile’ exists and is readable to the script. if [-r file ]; thencontent=$(cat file) # Set $content to the content of the filefi [ -s nonemptyfile ] file ‘nonemptyfile’ exists and has a size of more than 0 bytes. if [ -s logfile ]; thengzip logfile # Backup the old logfiletouch 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 ]; thenmysql –socket=/var/lib/mysql/mysql.sock # See this MySQL tipfi [ -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 ]; thenecho -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 ]; thenecho “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 ]; thengrub-install /dev/hdafi [ -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 ]; thenecho “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 ]; thenecho “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 ]; thencp -f localfile /mnt/remote/remotefile # Make sure the remote location has the newest version of the file, toofi [ same -ef file ] file ‘same’ and file ‘file’ refer to the same device/inode number. if [ /dev/cdrom -ef /dev/dvd ]; thenecho “Your primary cd drive appears to read dvd’s, too.”fi

2. String-based conditions: