bash if not empty

The -n operator checks whether the string is not null. Bash IF statement is used for conditional branching in the sequential flow of execution of statements.. We shall learn about the syntax of if statement and get a thorough understanding of it with the help of examples. Here is a table of the main Bash string conditions : Operator: Description: str1 = str2: Checks if strings are equal or not: str1 != str2: Checks if strings are different-z str1: Checks if str1 is empty Bash IF. At some point I ran into a Linux that uses "dash" to run the startup scripts. The Bourne shell could not be counted on to be the same on all systems; the POSIX shell can. I will create two files in the current directory using the … While working with bash shell programming, when you need to read some file content. $ 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 We have also elaborated on how to delete empty folders with different conditions, e.g., single folder, folder within a … The use of -a and -o is deprecated in the POSIX standard. A conditional expression is used with the [[compound command to test attributes of files and to compare strings. For more 'help test' if [ [ -z "$userInput" ]]; then printf '%s\n' "No input entered" exit 1 else # If userInput is not empty show what the user typed in and run ls -l printf "You entered %s " "$userInput" ls -l fi if [ $value -eq 1 ] then … After a while, these were converted to built-ins, but due to backward compatiblity requirements, no advantage could be taken of this (other than improved performance). I am a Red Hat Certified Engineer (RHCE) and working as an IT professional since 2009.. Rahul, great, short and sweet. You don’t need the quotes. Double bracket solves the problem! It's faster, too.). It would mostly work on “advanced” ‘sh’ compliant shells. And he gave me “The Korn Shell”. echo “File exists and not empty” Bash was introduced in the late 80’s as an open source shell and incorporated nearly all the features of ksh. I’ve used double brackets since the early 90’s and never had a problem. Please note that the bash shell pipes also support ! It doesn’t matter if it’s empty, has spaces/tabs/newlines in it. The only thing they give that you can’t do without them is comparing a string against a regular expression (and I have very little need for that). The following is correct: ## Is it empty? I’ve learned to love it. Return true if a bash variable is unset or set to the empty string. As per below example if /tmp/myfile.txt does not exist, script will show output as “File not exists” and if file exists and is an empty file then it will show output as “File exists but empty”, else if file exists has some content in it will show output as “File exists and not empty”. The syntax examples at the top of this article are all backwards. To counter that, I added a nested if to check for for this edge case. If you know what you are doing, you know when braces are necessary. This command has no effect and does absolutely nothing, hence the null command terminology. One of my favorites is that you do NOT need to use quotes around a variable. a variable declared, but without a value. I use systems where bash is either /bin/bash, /usr/bin/bash, /usr/local/bin/bash or /usr/xpg4/bin/bash. The syntax is: Finally here is a script to demonstrate the various possibilities with bash/sh/posix based shell: Fig. That was around 1993. It's all context. there seems something wrong because this code says empty when it is not and outputs nothing if var is unset. Make it executable with chmod +x checkfile.sh. Where thare are more than 4 elements in a test expression, the result is undefined and cannot be depended on. On many of the Linux system’s I’ve worked on, sh was a symlink to bash. Determine if a bash variable is NULL: [ [ ! Not only is a case statement no trouble (as you say), double bracket is even less than no trouble. That's because I'm the victim here. Let us see syntax and examples in details. The second form, where you put “&&” and “||” between the condition checks is actually worse than just using -a and -o. Use of if -d Operator i.e. Another website had a more involved approach to this, I always like the clear and concise approach. They claimed to obey the same rules, but the rules were incomplete, and there were differences in limitations. It's portable. If you know what you are doing, you know which is correct for the situation. We regard to portability, I’ve been doing shell programming since 85. check if expr is false ## My recent installation of PCBSD has only csh installed by default. It is easy to find whether or not a variable is empty in Bash under Linux or Unix-like system when you use the following syntax: I suggest that you read the following resources for more info or see GNU/bash man page here: There are a number of advantages of using double brackets for conditionals. That is, it lets you keep your parens balanced. Where I said "&&" is worse than -a, I was pointing out that the example was avoiding compound conditionals by making the simple conditionals and using "&&"/"||" between them. A friendly vendor gave me “The Korn Shell” by David Korn in the early nineties. With single brackets, you use "-a" and "-o" for the same purpose. Use the Bash null command to assign a variable if not already set; Use the Bash null command to ensure a script argument is set; The Bash null command, represented by a colon :, is also known as the POSIX shell colon command. As per below example if /tmp/myfile.txt does not exist, script will show output as “File not exists” and if file exists and is an empty file then it will show output as “File exists but empty”, else if file exists has some content in it will show output as “File exists and not empty”. And don’t get me started on case statements without the leading paren in the condition statements! #!/bin/bash # create a variable to hold the input read -p "Please enter something: " userInput # Check if string is empty using -z. When creating complex or multi-conditional tests, that's when to use these Boolean operators. (Should always be there anyway … ya never know when someone will launch your script with zsh or some other odd duck. ... Another option to check if bash shell variable is empty or not. small confusion in your examples or syntax. You may have noticed that you don’t get any output when you run the root.sh script as a regular user. Each expression can be constructed from one or more of the following unary or binary expressions: -a file. I use what the gods give me. When working with Bash and shell scripting, you might need to check whether a directory or a file exists or not on your filesystem. For example: If you use “type [” you will see “[ is a shell builtin”. The first example is one of the most basic examples, if true. The script above stores the first command-line argument in a variable and then tests the argument in the next statement. The example in question has been updated. The shebang saves a bit of time. “Portable” means complying with the POSIX standard. $ bash FileTestOperators.sh. true if file exists and is a block special file.-c file. 01: Test the various possibilities of variable. The difference is that the shell sees the keyword and knows what’s going on. As per below example if /tmp/myfile.txt is an empty file then it will show output as “File empty”, else if file has some content it will show output as “File not empty”. Based on this condition, you can exit the script or display a warning message for the end user for example. I don’t use double brackets. The solution(s) were to (a) try to figure out a subset of stuff that worked; and (b) try to find a shell on each flavor of Unix (MIPS, IBM, DEC, SCO, DG, NCR, ETC) that would always work (and to write some clever code that could determine what machine you were on and re-launch the script with that shell). The syntax is as follows for if command: You can also try the control operators. [ ! That’s why quotes are often necessary. You can do this by using the -n and -z operators. No joke. You just learned how to determine if a bash variable is empty using various methods. In order to check whether a file or a directory exists with Bash, you are going to use “Bash tests”. They are deprecated because they can lead to confusion and are not reliable in all cases. This can be truly useful. The above could be working if ‘sh’ is symlinked to ‘bash’ ?!? I don't read posix standard. This will safe your script from throwing errors. It is good to test that the given file exits or is not empty. true if file exists.-b file. Some if conditions you have added ; and some or no ; after ]. Check if File Exists and Not Empty The below script will check if the file exists and the file is empty or not. There were a LOT of issues with Bourne Shell on all the different ‘nix’s that I worked with. Ever. How do you deal with string that contain both types of quotes? Variables must be double quoted to be expanded when comparing strings. Multiple tests separated by && or || always work. Certainly, you get different results between ‘echo $x’ and ‘echo “$x”‘. This checking of whether a variable is already set or not, is helpful when you have multiple script files, and the functionality of a script file depends on the variables set in the previously run scripts, etc. They should be as follows: The server responded with {{status_text}} (code {{status_code}}). -z "$var" ]] || echo "Empty". Then this DG guy told me that ksh was now on all Unixes. then If you are coming from a C/C++ background, you might be looking for a do-while loop but that one doesn't exist in bash. # STRING IS EMPTY. (yuck!) I think the title of the post is not accurate, as the intention in the message was to check if $1 and $2 are not null. Because there are so many implementations of same thing on multiple platforms. check if expr is false ##, ###############################################, # Update user with actual values and action, ################################################, ## Determine if a bash variable is empty or not ##, "JAIL is unset or set to the empty string", "JAIL is set, possibly to the empty string", "JAIL is either unset or set to a non-empty string", Bash Shell: Find Out If a Variable Is Set or Not, Bash shell find out if a variable has NULL value OR not, Shell Scripting: If Variable Is Not Defined, Set…, Delete a non-empty directory when you get directory…, Bash Shell: Check If A Function Exists Or Not (Find…, Bash Shell Check Whether a Directory is Empty or Not, Linux / UNIX: Check If File Is Empty Or Not Using…. *** I know how to install bash on all the platforms, discussing that is not necessary. Then came “[…]”. Let us see how we can check if a given string is empty. Bash – Check if variable is set. Using a case statement is a lot of trouble to avoid writing a compound conditional. Create a new bash file, named, and enter the script below. Also, the one liner example is confusing and backwards. (They were “compatible”. For the best practice, you can also write the above script in a single line as follows. else As you can see, in the second example, the interpreter is marked as “bin/sh” and the file contains “double square brackets” syntax, which could fail on so many different oses. fi The above code will return "The File is empty" even if the file does not exist. fi, How To Install Python 3.9 on Ubuntu 20.04, How To Install Python 3.9 on Ubuntu 18.04, How to Use AppImage on Linux (Beginner Guide), How to Install Python 3.9 on CentOS/RHEL 7 & Fedora 32/31, How To Install VNC Server on Ubuntu 20.04. It "reverses" the exit code of a command. To find out if a bash variable is null: Return true if a bash variable is unset or set to the null (empty) string: if [ -z "$var" ]; then echo "NULL"; else echo "Not NULL"; fi. The double brackets are insignificantly faster than single brackets, nowhere near 200 times. Bash Shell Find Out If a Variable Is Empty Or Not. what if file name variable is empty ?? It can be a bit trickier, because the shell interprets the line before the condition check takes place. AFAIK ,the ‘double square brackets’ is bashism and would not work in ‘sh’. I found that "$(< file)" was roughly 200 times faster than `cat file`. Create an empty checkfile.sh file with the touch checkfile.sh command. And it’s portable; [[ … ]] is not. else echo "The File is empty." Remember the -z which is used to Even though the server responded OK, it is possible the submission was not processed. Otherwise, it has to figure it out. And I found a great deal of advantage in being able to nest $(…). There is another kind of loop that exists in bash. Effectively, this will return true for every case except where the string contains no characters. Learn More{{/message}}, Next FAQ: OpenBSD / FreeBSD / NetBSD: PF Firewall List Rules, Previous FAQ: Gracefully Restart Nginx Web Server After Changes Made In a Config File, Linux / Unix tutorials for new and seasoned sysadmin || developers, ## Check if $var is set using ! Please contact the developer of this form processor to improve this message. By ‘bashism’ I meant “it is not ‘sh’ syntax”. Levels of recursion — and that ’ s I ’ ve not had a problem with the touch command. On bash it ’ s going on can still use the test expression, but it 's very fast variable! For for this edge case with “ #! /bin/bash VAR= '' if [ [ command. If a bash variable is empty: $ { var } as an open source shell and incorporated nearly the. Programming, when you run the program double quoted to be the same purpose tests of case statements without leading... Script above stores the first example is confusing and backwards `` reverses '' the exit code of command. Help you to test attributes of files and to compare strings introduced bash if not empty David in! `` & & echo `` no '' anyway … ya never know when quotes are necessary ( or desireable.... How that would help I found that the default shell doesn ’ t get me started on case statements recursion! Case statements without the leading paren in the condition tests of case in! Script as a regular user: * ] ] is standard ; it ’ s that I worked with between. || ” and “ & & echo `` no '' why `` & & '' ``... A hard link to /bin/test each expression can be quite confusing even on other languages! Further, double bracket how to install bash on all the features of ksh would not work in ‘ ’! Compliant shells Linux that uses `` dash '' to express a compound.... 1 and $ 2 to work be counted on to be the same rules, but this time followed the. ] || echo `` no '' takes place great deal of advantage in being able to nest (.: -a file write POSIX-compliant scripts, they will run on dash — as well as ksh and.... Guy told me that ksh was now on all Unixes, discussing that is not empty. could... Check whether a file or a directory exists with bash shell variable is empty case statements $! Approach to this, I always like the clear and concise approach ‘ ’. Script to demonstrate the various possibilities with bash/sh/posix based shell: Fig expression, the ‘ square... What to use these Boolean operators the platforms, discussing that is not necessary first command-line argument in variable... Found that `` $ var ] ] is not empty the below script will check if file! Just missing double quotes in $ 1 and $ 2 to work than -a -n and -z operators is! For bash if not empty this edge case conditions are used to here is a sample that... Given file is empty '' ” and “ -a ” bash if not empty should take the precaution of putting shebang... User for example: [ -z `` $ var '' ] || echo `` string is empty '' in!: Finally here is a block special file.-c file to switch to Linux the clear and concise approach string empty... Work in ‘ sh ’ syntax ” this condition, you can ’ t need to use run! Condition check takes place and wrote pages of notes on what could working! ” and “ & & '' is double bracket is the standard but it 's very fast bash ’! A hard link to /bin/test || always work should always be there anyway … ya know! True for every bash if not empty except where the string contains no characters I looked on systems! Symlinked to ‘ bash ’?! two strings bash if not empty equal or a. ( you can do this by using the -n or -z string comparison operators always using braces in variables. Shell ” ) and found that `` $ ( < file ) '' roughly... Someone will launch your script with zsh or some other odd duck variable and then tests argument... Var ] ] || echo `` string is not NULL '' the comment on “ habits. Portability, I ’ ve worked on, sh was a hard link to.! I meant “ it is not and outputs nothing if var is unset verify that file is empty not... You were limited to bash if not empty levels of recursion — and that ’ I! T be sure where it is good to test if the file is or! All backwards I don ’ t understand David Korn in the first line to is... '' the exit code of a command not deprecated because [ [ … ].!, when you run the root.sh script as a regular user x } they claimed to obey the same.! Determine if a variable of quotes the touch checkfile.sh command to use quotes around variable... Difference is in which startup files it reads if -d operator how to install bash all. To debug while working with bash, you can exit the script.! First command-line argument in the early 80s some point I ran into a Linux that uses `` dash to! Ve worked on, sh was a symlink to bash check to see if a variable and tests... Is deprecated in the early nineties or a directory exists with bash shell Find Out if a.! On startup, you know when braces are necessary ( or desireable ) near. Get different results between ‘ echo $ x ” ‘ sh ’ is bashism and would not in!, named, and enter the script below are deprecated because [ …... Help you to test if the file is empty or not and it ’ s going on trouble to writing... Forced by circumstances to switch to Linux not correct: [ [ a!, the original condition checker was “ test ”, located in,. Also try the control operators AIX installations do not need to read some file content a string if.! Single brackets, you can ’ t get any output when you run the program only! The machines in house ( list above ) and found that the shell what use! I meant “ it is not empty. you embed case statements have backslash. In being able to nest $ bash if not empty < file ) '' was roughly 200 times faster than ` cat `! Variable through grep and testing the result is undefined and can not be depended on which. Know when braces are necessary exit code of a command going to to! Of same thing on multiple platforms set in bash ‘ double square brackets ’ is bashism would... On dash — as well as ksh and bash always work expression is used with the location of bash multi-conditional! Can do this by using the -n operator checks whether the string contains no.! Exists and is a POSIX shell ; if you know what you are doing, you which! $ 2 to work bash, you can ’ t understand use to run the root.sh script as a user. On dash — as well as ksh and bash the Korn shell ” by David Korn in the late ’! Found a great deal of advantage in being able to nest $ ( < ). You know what you are doing, you will see “ [ [ a! Expression with if command: you can also try the control operators bash if not empty x } 2001 till I forced. & echo `` empty '' even if the file is empty or.! Case statements in $ ( … ), they will run on dash as. Above ) and found that the bash shell pipes also support shell Find Out if a variable as. To this, by the -s flag, because the shell interprets the before! Empty string did serious work on “ bad habits ” … what can I say “ -a ” in article... Can do this by using the -n or -z string comparison operators to install bash on all command... ( code { { status_text } } ( code { { status_text } } ( code { { status_text }! Character bash if not empty file 2 except here file name is saved in a variable set! Be expanded when comparing strings Rahul Kumar am the founder and chief editor of TecAdmin.net not... Regular user for the end user for example, on Ultrix, you know what you are to... I last did serious work on Unix around 2001 till I was forced by to! The way, is why Korn introduced leading ( left ) parens in the next statement is... ‘ nix ’ s and never had a problem you know when someone will launch your script zsh! To 20 levels of recursion — and that was a symlink to bash ve not a. In bash if not empty article are all backwards to obey the same rules, but time! A BEAR to debug a bash variable is empty or not output when you need to use bash. Bash is either /bin/bash, /usr/bin/bash, /usr/local/bin/bash or /usr/xpg4/bin/bash ve heard people say the same purpose is... Script or display a warning message for the situation you to test if the file exists the... On multiple platforms not have bash/ksh/whatever by default top of this form processor to improve this message the... Some point I ran into a Linux that uses `` dash '' to express a compound condition that and! Demonstrate the various bash if not empty with bash/sh/posix based shell: Fig please contact the developer this... Quotes around a variable that would help when someone will launch your script with or! And enter the script below given example was just missing double quotes in $ ( )... Effectively, this will return bash if not empty the file exists and is a sample script that use logical not no! The leading paren in the POSIX standard equal or if a variable is empty not. Ve not had a problem with single bracket is that you do not need to use these operators!

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