Note that if the standard input is not empty, it is passed through ifne to the given command A simple solution is to use ifne command (if input not empty). If the command (at -l) command returns no value (is empty/null) then write a message to the file in place of the command output. #!/bin/bash if [ $(whoami) = 'root' ]; then echo "You are root" fi. @echo off setlocal rem Subversion sends through the path to the repository and transaction id set REPOS=%1 set TXN=%2 rem check for an empty log message svnlook log %REPOS% -t %TXN% | findstr . Note that if the standard input is not empty, it is passed through ifne in this case. Create a new bash file, named, and enter the script below. DESCRIPTION ifne runs the following command if and only if the standard input is not empty. Check to see if a variable is empty or not. You can quickly test for null or empty variables in a Bash shell script. test provides no output, but returns an exit status of 0 for "true" (test successful) and 1 for "false" (test failed). Several other tests allow you to check things such as the permissions of the file. ifne runs a given command if and only if the standard input is not empty. check_empty.sh and execute with our without command line arguments: $ bash check_empty.sh Empty Variable 1 Empty Variable 3 Empty Variable 5 Furthermore, the execution of the above script with a command line argument will trigger opposite results: $ bash check_empty.sh hello Not Empty Variable 2 Not Empty Variable 4 Not empty Variable 5 The -s option to the test builtin check to see if FILE exists and has a size greater than zero. The whoami command outputs the username. OPTIONS -n Reverse operation. In this case you can omit the 2 from the redirection, but you will lose any output from the command. How do I check if a file is empty in Bash? The condition $(whoami) = 'root' will be true only if you are logged in as the root user. EXAMPLE find . From the bash variables tutorial, you know that $(command) syntax is used for command substitution and it gives you the output of the command. Checking $? Bash/Shell: Check if file exists (is empty or not empty) To check if the file exists and if it is empty or if it has some content then we use "-s" attribute . You can use the help command for other builtins too.. returns success (0) as the command ran but the return from the command is empty. 2.1: Method-1: Using single or double brackets ... We can use find command and then suppress the output to check if the target directory has some content or not. Appreciate any help, thanks. -name core | ifne mail … The test command is frequently used as part of a conditional expression . See the man pages for bash for more details or use help test to see brief information on the test builtin. You need to pass the -z or -n option to the test command or to the if command or use conditional expression.This page shows how to find out if a bash shell variable has NULL value or not using the test command. In some distributions, it is not installed by default. > nul if %errorlevel% gtr 0 (goto err) else exit 0 :err echo. H ow do I check if a file is empty or not using bash or ksh shell script under a UNIX / Linux / macOS / OS X / BSD family of operating systems? My problem is around trapping the empty returned command value and replacing with my own message. Also, since any bash test without an argument will always return true, it is advisable to quote the variables so that the test will have at least an (empty) argument, irrespective of whether the variable is set or not. Run the command if the standard input is emp- ty. To test for whitespace characters, the $'…' syntax is specific to ksh/bash/zsh; you can insert these characters in your script literally (note that a newline will have to be within quotes, as backslash+newline expands to nothing), or generate them, e.g. If command writes errors out to stderr, you can use the form command 2> /dev/null && echo OK || echo Failed.The 2> /dev/null redirects stderr output to /dev/null.However, some utilities will write errors to stdout (even though this is bad practice). It is a part of the package moreutils in most distros. For instance, the following statement says, "If 4 is greater than 5, output yes, otherwise output no." You can use the find command and other options as follows. Part of a conditional expression returns success ( 0 ) as the bash check if output of command is empty... The package moreutils in most distros are root '' fi option to the test command is frequently used as of! 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