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• 12 Conditional Expressions

12 Conditional Expressions

A conditional expression is used with the [[ compound command to test attributes of files and to compare strings. Each expression can be constructed from one or more of the following unary or binary expressions:

-a file
true if file exists.

-b file
true if file exists and is a block special file.

-c file
true if file exists and is a character special file.

-d file
true if file exists and is a directory.

-e file
true if file exists.

-f file
true if file exists and is a regular file.

-g file
true if file exists and has its setgid bit set.

-h file
true if file exists and is a symbolic link.

-k file
true if file exists and has its sticky bit set.

-n string
true if length of string is non-zero.

-o option
true if option named option is on. option may be a single character, in which case it is a single letter option name. (See Specifying Options.)

When no option named option exists, and the POSIX_BUILTINS option hasn’t been set, return 3 with a warning. If that option is set, return 1 with no warning.

-p file
true if file exists and is a FIFO special file (named pipe).

-r file
true if file exists and is readable by current process.

-s file
true if file exists and has size greater than zero.

-t fd
true if file descriptor number fd is open and associated with a terminal device. (note: fd is not optional)

-u file
true if file exists and has its setuid bit set.

-v varname
true if shell variable varname is set.

-w file
-x file
-z string
true if length of string is zero.

-L file
true if file exists and is a symbolic link.

-O file
true if file exists and is owned by the effective user ID of this process.

-G file
true if file exists and its group matches the effective group ID of this process.

-S file
true if file exists and is a socket.

-N file
true if file exists and its access time is not newer than its modification time.

file1 -nt file2
true if file1 exists and is newer than file2.

file1 -ot file2
true if file1 exists and is older than file2.

file1 -ef file2
true if file1 and file2 exist and refer to the same file.

string = pattern
string == pattern
true if string matches pattern. The two forms are exactly equivalent. The ‘=’ form is the traditional shell syntax (and hence the only one generally used with the test and [ builtins); the ‘==’ form provides compatibility with other sorts of computer language.

string != pattern
true if string does not match pattern.

string =~ regexp
true if string matches the regular expression regexp. If the option RE_MATCH_PCRE is set regexp is tested as a PCRE regular expression using the zsh/pcre module, else it is tested as a POSIX extended regular expression using the zsh/regex module. Upon successful match, some variables will be updated; no variables are changed if the matching fails.

If the option BASH_REMATCH is not set the scalar parameter MATCH is set to the substring that matched the pattern and the integer parameters MBEGIN and MEND to the index of the start and end, respectively, of the match in string, such that if string is contained in variable var the expression ‘${var[$MBEGIN,$MEND]}’ is identical to ‘$MATCH’. The setting of the option KSH_ARRAYS is respected. Likewise, the array match is set to the substrings that matched parenthesised subexpressions and the arrays mbegin and mend to the indices of the start and end positions, respectively, of the substrings within string. The arrays are not set if there were no parenthesised subexpressions. For example, if the string ‘a short string’ is matched against the regular expression ‘s(...)t’, then (assuming the option KSH_ARRAYS is not set) MATCH, MBEGIN and MEND are ‘short’, 3 and 7, respectively, while match, mbegin and mend are single entry arrays containing the strings ‘hor’, ‘4’ and ‘6’, respectively.

If the option BASH_REMATCH is set the array BASH_REMATCH is set to the substring that matched the pattern followed by the substrings that matched parenthesised subexpressions within the pattern.

string1 < string2
true if string1 comes before string2 based on ASCII value of their characters.

string1 > string2
true if string1 comes after string2 based on ASCII value of their characters.

exp1 -eq exp2
true if exp1 is numerically equal to exp2. Note that for purely numeric comparisons use of the ((...)) builtin described in Arithmetic Evaluation is more convenient than conditional expressions.

exp1 -ne exp2
true if exp1 is numerically not equal to exp2.

exp1 -lt exp2
true if exp1 is numerically less than exp2.

exp1 -gt exp2
true if exp1 is numerically greater than exp2.

exp1 -le exp2
true if exp1 is numerically less than or equal to exp2.

exp1 -ge exp2
true if exp1 is numerically greater than or equal to exp2.

( exp )
true if exp is true.

! exp
true if exp is false.

exp1 && exp2
true if exp1 and exp2 are both true.

exp1 || exp2
true if either exp1 or exp2 is true.

For compatibility, if there is a single argument that is not syntactically significant, typically a variable, the condition is treated as a test for whether the expression expands as a string of non-zero length. In other words, [[ $var ]] is the same as [[ -n $var ]]. It is recommended that the second, explicit, form be used where possible.

Normal shell expansion is performed on the file, string and pattern arguments, but the result of each expansion is constrained to be a single word, similar to the effect of double quotes.

Filename generation is not performed on any form of argument to conditions. However, it can be forced in any case where normal shell expansion is valid and when the option EXTENDED_GLOB is in effect by using an explicit glob qualifier of the form (#q) at the end of the string. A normal glob qualifier expression may appear between the ‘q’ and the closing parenthesis; if none appears the expression has no effect beyond causing filename generation. The results of filename generation are joined together to form a single word, as with the results of other forms of expansion.

This special use of filename generation is only available with the [[ syntax. If the condition occurs within the [ or test builtin commands then globbing occurs instead as part of normal command line expansion before the condition is evaluated. In this case it may generate multiple words which are likely to confuse the syntax of the test command.

For example,

[[ -n file*(#qN) ]]

produces status zero if and only if there is at least one file in the current directory beginning with the string ‘file’. The globbing qualifier N ensures that the expression is empty if there is no matching file.

Pattern metacharacters are active for the pattern arguments; the patterns are the same as those used for filename generation, see Filename Generation, but there is no special behaviour of ‘/’ nor initial dots, and no glob qualifiers are allowed.

In each of the above expressions, if file is of the form ‘/dev/fd/n’, where n is an integer, then the test applied to the open file whose descriptor number is n, even if the underlying system does not support the /dev/fd directory.

In the forms which do numeric comparison, the expressions exp undergo arithmetic expansion as if they were enclosed in $((...)).

For example, the following:

[[ ( -f foo || -f bar ) && $report = y* ]] && print File exists.

tests if either file foo or file bar exists, and if so, if the value of the parameter report begins with ‘y’; if the complete condition is true, the message ‘File exists.’ is printed.

This document was generated on May 14, 2022 using texi2html 5.0.
Zsh version 5.9, released on May 14, 2022.