| A conditional expression is used with the [[compound command to test attributes of files and to compare strings. Field splitting and file name generation are not performed on the words between [[ and ]]. Each expression can be constructed from one or more of the following unary or binary expressions: | string | True, if string is not null. | | -a file | Same as -e below. This is obsolete. | | -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 an ordinary file. | | -g file | True, if file exists and it has its setgid bit set. | | -k file | True, if file exists and it has its sticky bit set. | | -n string | True, if length of string is non-zero. | | -o option | True, if option named option is on. | | -p file | True, if file exists and is a fifo special file or a 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 fildes | True, if file descriptor number fildes is open and associated with a terminal device. | | -u file | True, if file exists and it 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 true if 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. | file1 -nt file2 | True, if file1 exists and file2 does not, or file1 is newer than file2. | file1 -ot file2 | True, if file2 exists and file1 does not, or file1 is older than file2. | file1 -ef file2 | True, if file1 and file2 exist and refer to the same file. | string == pattern | True, if string matches pattern. Any part of pattern can be quoted to cause it to be matched as a string. | string = pattern | Same as == above, but is obsolete.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. |
The following obsolete arithmetic comparisons are also permitted: exp1 -eq exp2 | True, if exp1 is equal to exp2. | exp1 -ne exp2 | True, if exp1 is not equal to exp2. | exp1 -lt exp2 | True, if exp1 is less than exp2. | exp1 -gt exp2 | True, if exp1 is greater than exp2. | exp1 -le exp2 | True, if exp1 is less than or equal to exp2. | exp1 -ge exp2 | True, if exp1 is greater than or equal to exp2. |
In each of the above expressions, if file is of the form /dev/fd/n, where n is an integer, then the test is applied to the open file whose descriptor number is n. A compound expression can be constructed from these primitives by using any of the following, listed in decreasing order of precedence. (expression) | True, if expression is true. Used to group expressions. | ! expression | True if expression is false. | expression1 && expression2 | True, if expression1 and expression2 are both true. | expression1 || expression2 | True, if either expression1 or expression2 is true. |
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