#Bash grep output file timestamp portable#
#Bash grep output file timestamp code#

] # Do I really need to say it…Īs explained in the previous example, [ is a command (which can be proven with type -t [ or whence -v [). Otherwise, if there are no matches, grep will return non-zero and the overall if command will be zero.

#Bash grep output file timestamp code#

If the grep matches a line from myfile, then the exit code will be 0 (true), and the then part will be executed. If you want to make a decision based on the output of a grep command, you do not want to enclose it in parentheses, brackets, backticks, or any other syntax! Just use grep as the COMMANDS after the if, like this: If no then section is evaluated, then the else branch is taken, or if no else is given, the if block is complete and the overall if command returns 0 (true). Another elif is evaluated until one of the then sections is executed. The exit status of the final command of the first section and each subsequent elif section determines whether each corresponding then section is evaluated.

The if compound command is made up of two or more sections containing lists of commands, each delimited by a then, elif, or else keyword, and is terminated by the fi keyword. There may be zero or more optional elif sections, and one optional else section. Bash only passes ] as argument to the to be the last argument only to make scripts look better. if is a compound command which contains other commands - and there is no [ in its syntax! It takes arguments like any other regular simple command. In both cases HELP will always be printed, to the surprise of programmers from other languages guessing about shell syntax. For example:Īre equivalent - both checking that the argument 'false' is non-empty. This is not the case! if takes a command. This convinces people that the [ is somehow part of the if statement's syntax, just like parentheses used in C's if statement. Many beginners have an incorrect intuition about if statements brought about by seeing the very common pattern of an if keyword followed immediately by a [ or [[.

#Bash grep output file timestamp portable#

Therefore, portable scripts must be written in such a way as to not depend upon either behavior.įor workarounds for this and similar issues, please see Bash FAQ #24. Some shells such as ksh93 and Bash >= 4.2 with shopt -s lastpipe enabled will run the while loop in this example in the original shell process, allowing any side-effects within to take effect. POSIX doesn't specify whether or not the last element of a pipeline is evaluated in a subshell. This surprises almost every Bash beginner at some point. Changes to count won't propagate outside the while loop because each command in a pipeline is executed in a separate SubShell. The code above looks OK at first glance, doesn't it? Sure, it's just a poor implementation of grep -c, but it's intended as a simplistic example. grep foo bar | while read -r do ((count++)) done Note that as would return true, while ] and (( 010 > 8 )) would return false.Ĩ. Printf '$foo is not a sequence of decimal digits: '%s' ' '$foo' >&2 If the input to any arithmetic context (including ((, let, array indices), or ] test expressions involving numeric comparisons can't be guaranteed then you must always validate your input before evaluating the expression. If the contents of $foo are not sanitised and are out of your control (if for instance they're coming from an external source), then all but constitute an arbitrary command injection vulnerability as the contents of $foo are interpreted as an arithmetic expression (and for instance, the a arithmetic expression would run the reboot command when evaluated). # POSIX-compatible equivalent to ((, for more general math operations. If strict POSIX-conformance is a requirement, and (( is not available, then the correct alternative using [ is You'll get a file named 7 in your directory, and the test will succeed as long as $foo is not empty. If you use > inside, it's even worse: it's an output redirection.

This may work sometimes, but it will fail when you least expect it. If you use the > operator inside ], it's treated as a string comparison (test for collation order by locale), not an integer comparison. If you just want to do a numeric comparison (or any other shell arithmetic), it is much better to just use (( )) instead: Though technically you can do math using some of [['s operators, it only makes sense to do so in conjunction with one of the non-math test operators somewhere in the expression. It should be used for test expressions involving one of the supported test operators. First, the [[ command should not be used solely for evaluating arithmetic expressions. grep foo bar | while read -r do ((count++)) done.