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11. 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 15.1 Specifying Options.)

-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.

-w file
true if file exists and is writable by current process.

-x file
true if file exists and is executable by current process. If file exists and is a directory, then the current process has permission to search in the directory.

-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 `==' form is the preferred one. The `=' form is for backward compatibility and should be considered obsolete.

string != pattern
true if string does not match 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.

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.

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. However, pattern metacharacters are active for the pattern arguments; the patterns are the same as those used for filename generation, see 13.8 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.

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This document was generated by Peter Stephenson on August, 9 2002 using texi2html