You can do this by using the -n and -z operators. If you know what you are doing, you know when braces are necessary. It let you embed case statements in $(…) command line substitutions. Create an empty checkfile.sh file with the touch checkfile.sh command. Lets create a bash script, that will check whether a passed as an argument file exists or not, by printing a corresponding message. And if you use a non-standard shell, you can’t be sure where it is. fi, How To Install Python 3.9 on Ubuntu 20.04, How To Install Python 3.9 on Ubuntu 18.04, How to Use AppImage on Linux (Beginner Guide), How to Install Python 3.9 on CentOS/RHEL 7 & Fedora 32/31, How To Install VNC Server on Ubuntu 20.04. For more 'help test' if [ [ -z "$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 Check If The File is Not Empty Sometimes, after verifying that a regular file exists, we might also want to check if that file is not empty. # Check if $var is set using ! Let us see syntax and examples in details. Another website had a more involved approach to this, I always like the clear and concise approach. As you can see, in the second example, the interpreter is marked as “bin/sh” and the file contains “double square brackets” syntax, which could fail on so many different oses. Open the checkfile.sh with a … The -n operator checks whether the string is not null. We regard to portability, I’ve been doing shell programming since 85. There were a LOT of issues with Bourne Shell on all the different ‘nix’s that I worked with. For example, on Ultrix, you were limited to 20 levels of recursion — and that was a BEAR to debug! That’s why quotes are often necessary. They are deprecated because they can lead to confusion and are not reliable in all cases. As per below example if /tmp/myfile.txt is an empty file then it will show output as “File empty”, else if file has some content it will show output as “File not empty”. I’ve used double brackets since the early 90’s and never had a problem. Because there are so many implementations of same thing on multiple platforms. Using if-else statement in bash. (yuck!) To counter that, I added a nested if to check for for this edge case. Where thare are more than 4 elements in a test expression, the result is undefined and cannot be depended on. If you get no output then the directory is empty or else not empty #!/bin/bash DIR =" /tmp " if [ [ -z "$ (ls -A -- "$DIR")" ]] ; then echo … One of my favorites is that you do NOT need to use quotes around a variable. There are several reasons Korn added double bracket. true if file exists and is a character special file. The double brackets are insignificantly faster than single brackets, nowhere near 200 times. 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. This script will print the first argument because it is not empty. Bash IF. It’s hard to know, since by convention, shell scripts have the “shebang” as the first line (#!/bin/bash). Based on this condition, you can exit the script or display a warning message for the end user for example. Check to see if a variable is empty or not. AFAIK ,the ‘double square brackets’ is bashism and would not work in ‘sh’. Bash – set default value if a variable is empty By admin on Dec 16, 2015 There are multiple ways to check if a variable is empty and then set it to some default value. Bash string conditions are used to check if two strings are equal or if a string if empty. All I was saying was … the sentence should clearly state why the “double-square-bracket” syntax could fail, rather than just mentioning “portability”. Use of if -d Operator How to check if a Bash Array is empty? echo “File not exists” I, Rahul Kumar am the founder and chief editor of TecAdmin.net. Using && in an IF statement in bash. *** I know how to install bash on all the platforms, discussing that is not necessary. else A friendly vendor gave me “The Korn Shell” by David Korn in the early nineties. Further, double bracket knows what “||” and “&&” are. 12 Conditional Expressions. The shebang saves a bit of time. I think the title of the post is not accurate, as the intention in the message was to check if $1 and $2 are not null. If you know what you are doing, you know which is correct for the situation. (It tells the shell what to use to run the program. -z "$var" ]] || echo "Empty". I don’t use double brackets. Many-a-times, AIX installations do not have bash/ksh/whatever by default. #!/bin/bash VAR='' if [ [ -z $VAR ]]; then echo "String is empty." The second form, where you put “&&” and “||” between the condition checks is actually worse than just using -a and -o. The syntax is: Finally here is a script to demonstrate the various possibilities with bash/sh/posix based shell: Fig. That's because I'm the victim here. echo “File exists but empty” That is, it lets you keep your parens balanced. When bash is started as sh, the only difference is in which startup files it reads. For the best practice, you can also write the above script in a single line as follows. -h. file is a symbolic link-L. file is a symbolic link-S. file is a socket-t. file is associated with a terminal deviceThis test option may be used to check whether the stdin [ -t 0 ] or stdout [ -t 1 ] in a given script is a terminal.-r. file has read permission (for the user running the test)-w. file has write permission (for the user running the test) #!/bin/bash # create a variable to hold the input read -p "Please enter something: " userInput # Check if string is empty using -z. ie: And that’s why variables need not be quoted. The solution(s) were to (a) try to figure out a subset of stuff that worked; and (b) try to find a shell on each flavor of Unix (MIPS, IBM, DEC, SCO, DG, NCR, ETC) that would always work (and to write some clever code that could determine what machine you were on and re-launch the script with that shell). NOT!) [ -z "$var" ] && echo "Empty" I looked on all the machines in house (list above) and found that it was true. Let us see how we can check if a given string is empty. You may have noticed that you don’t get any output when you run the root.sh script as a regular user. This will safe your script from throwing errors. How do you deal with string that contain both types of quotes? The above could be working if ‘sh’ is symlinked to ‘bash’ ?!? Any code you want to run when an if condition is evaluated to false can be included in an else statement as follows: #!/bin/bash if [ $(whoami) = 'root' ]; then echo "You are root" else echo "You are not root" fi true if file exists.-b file. In this example, the variable count specifies a condition that is used as part of the if statement.Before the if statement is executed, the variable count is assigned the value 5.The if statement then checks whether the value of count is 5.If that is the case, the statement between the keywords then and fi are executed.Otherwise, any statements following the if statement are executed. I’ve learned to love it. I’ve adopted all the new features, and (due to other things), got as much as a 200 times performance improvement over the old Bourne Shell. what if file name variable is empty ?? Example: else echo "The File is empty." Otherwise, it has to figure it out. i.e. Again as I said since there is no easy way to check if a directory is empty or not using some attributes, we will list the content of the directory. Regardless, these are equivalents. To sum up, we have successfully discussed the methods on how to delete the empty and non-empty folders in Bash using the “rmdir” and “rm” command, respectively. Variables must be double quoted to be expanded when comparing strings. -z "$var" ] || echo "Empty" With single brackets, you use "-a" and "-o" for the same purpose. With double brackets, you use "&&" and "||" to express a compound condition. Still it says File exists and not empty, if [ -f ${FILENAME} ];then Actually ‘[‘ was a hard link to /bin/test. I don’t think there are any nix’s that don’t have bash or ksh. It can be a bit trickier, because the shell interprets the line before the condition check takes place. I’ve been using Linux almost exclusively for the last ten or so years, but have occasionally used other ‘nix’s (most recently AIX). Make it executable with chmod +x checkfile.sh. Builtins act like programs. While working with bash shell programming, when you need to read some file content. Syntax of if statement Using a case statement is a lot of trouble to avoid writing a compound conditional. This article will help you to test if the file exists or not and the file is empty or not. 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. 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”. On many of the Linux system’s I’ve worked on, sh was a symlink to bash. The difference is that the shell sees the keyword and knows what’s going on. The problem with single bracket is that all the command line parsing is completed before the condition is tested. If value equals 1. a variable is considered empty if it doesn't exist or if its value is one of the following: (an empty string) 0 (0 as an integer) 0.0 (0 as a float) 0 (0 as a string) an empty array. To check if a variable is set in Bash Scripting, use-v var or-z ${var} as an expression with if command.. It "reverses" the exit code of a command. The until loop follows the same syntax as the while loop: until [ condition ]; do [COMMANDS] Done The comment on “bad habits” … what can I say? Dash is a minimal shell, but it's very fast. You can also try the control operators. Certainly, you get different results between ‘echo $x’ and ‘echo “$x”‘. The example in question has been updated. It's all context. I found ksh this way. Fig.01: Bash scripting various way to determine if a variable is empty. The shebang is unnecessary unless the script uses syntax that the default shell doesn’t understand. You can also use the following two commands to verify that file is empty: $ cat Test.txt $ file Test.txt. I use what the gods give me. String comparison can be quite confusing even on other programming languages. if [ -s ${FILENAME} ] A case statement is no trouble and is much more flexible than [[ … ]]. Its features are unchanged. [[ -f $File && -r $File ]] || echo “‘Not a readable file: ‘$File'”, Finally, as noted before, you can compare strings (or variable) to shell patterns (not regular expressions). With an exception. The Bourne shell could not be counted on to be the same on all systems; the POSIX shell can. You don’t need the quotes. And I found a great deal of advantage in being able to nest $(…). 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. But here on bash it’s quite straight forward. NOTE: THE EXAMPLES IN THIS ARTICLE ARE NOT CORRECT. Determine if a bash variable is NULL: [ [ ! 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”. Please contact the developer of this form processor to improve this message. It knows that $var is a variable. I last did serious work on Unix around 2001 till I was forced by circumstances to switch to Linux. In order to check whether a file or a directory exists with Bash, you are going to use “Bash tests”. Let us see syntax and examples in details. A conditional expression is used with the [[compound command to test attributes of files and to compare strings. #!/bin/bash if [ -s aFile ]; then echo "The File has some data." I will create two files in the current directory using the … Many non-Linux nixes have only sh (which may or may not be some variant of ksh on solaris, others, I dunno). Bash if variable is not empty. It's portable. To do that we can still use the test expression, but this time followed by the -s flag. I don't read posix standard. After a while, these were converted to built-ins, but due to backward compatiblity requirements, no advantage could be taken of this (other than improved performance). “Portable” means complying with the POSIX standard. [ ! I found that the pattern matching ability of double-bracket was roughly 200 times faster than echoing a variable through grep and testing the result. The first example is one of the most basic examples, if true. Your email address will not be published. My recent installation of PCBSD has only csh installed by default. To find out if a bash variable is null: Return true if a bash variable is unset or set to the null (empty) string: if [ -z "$var" ]; then echo "NULL"; else echo "Not NULL"; fi. i.e. 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…. You can compare strings or variables against a pattern without using double brackets;use a case statement: You don’t need double brackets to use && or || with test: Having to quote variables is not a problem; not doing it leads to bad habits. Please contact the developer of this form processor to improve this message. It is good to test that the given file exits or is not empty. I am a Red Hat Certified Engineer (RHCE) and working as an IT professional since 2009.. Rahul, great, short and sweet. For example, I found that using ((Idx = 0; Idx < $X; Idx++)) was roughly 200 times faster than using "Idx=`expr $Idx + 1`" in a loop and then testing $Idx -ge $X. Only sh is installed. So, just how "non-portable" is double bracket? fi Here is a sample script that use logical not ! Then came “[…]”. The following is correct: ## Is it empty? Then this DG guy told me that ksh was now on all Unixes. Double brackets were introduced by David Korn in ksh in the early 80s. Create a new bash file, named, and enter the script below. When creating complex or multi-conditional tests, that's when to use these Boolean operators. Bash was introduced in the late 80’s as an open source shell and incorporated nearly all the features of ksh. a variable declared, but without a value. This command has no effect and does absolutely nothing, hence the null command terminology. [[ ! And don’t get me started on case statements without the leading paren in the condition statements! In Bash you quite often need to check to see if a variable has been set or has a value other than an empty string. They claimed to obey the same rules, but the rules were incomplete, and there were differences in limitations. Quite often you will also need to check whether a variable is an empty string or not. Until Loops in Bash. We have also elaborated on how to delete empty folders with different conditions, e.g., single folder, folder within a … And, no, -a and -o are not deprecated because [[ … ]] is standard; it’s NOT. At some point I ran into a Linux that uses "dash" to run the startup scripts. If you remember your history, the original condition checker was “test”, located in /bin. OpenBSD / FreeBSD / NetBSD: PF Firewall List Rules, Gracefully Restart Nginx Web Server After Changes Made In a Config File, 30 Cool Open Source Software I Discovered in 2013, 30 Handy Bash Shell Aliases For Linux / Unix / Mac OS X, Top 32 Nmap Command Examples For Linux Sys/Network Admins, 25 PHP Security Best Practices For Linux Sys Admins, 30 Linux System Monitoring Tools Every SysAdmin Should Know, Linux: 25 Iptables Netfilter Firewall Examples For New SysAdmins, Top 20 OpenSSH Server Best Security Practices, Top 25 Nginx Web Server Best Security Practices. there seems something wrong because this code says empty when it is not and outputs nothing if var is unset. I'm gonna guess, however, that -a and -o are deprecated because double bracket is the standard. Check if File Exists and Not Empty The below script will check if the file exists and the file is empty or not. No joke. Bash – Check if variable is set. Korn introduced double brackets to get around the backward-compatibility requirement. Some if conditions you have added ; and some or no ; after ]. Double bracket solves the problem! [[ $S3Msg = ERROR:* ]] && S3Stat=1. That’s the point. I would highly recommend you do indent your code however (especially as your scripts get larger) otherwise you will find it … Another option to find if bash variable set to NULL: [ -z "$var" ] && echo "NULL". The given example was just missing double quotes in $1 and $2 to work. You can check if an array is empty by checking the length (or size) of the array with the $ {#array [@]} syntax and use a bash if statement as necessary. Even though the server responded OK, it is possible the submission was not processed. then 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 syntax is as follows for if command: You can also try the control operators. As the file is not empty this time, so the output generated by the -s operator will be True as shown in the image. [[ -z "$var" ]] && echo "Empty" small confusion in your examples or syntax. Also, the one liner example is confusing and backwards. 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 ! This can be truly useful. Return true if a bash variable is unset or set to the empty string. So, you don’t need to use “-o” and “-a”. You know when quotes are necessary (or desireable). Not only is a case statement no trouble (as you say), double bracket is even less than no trouble. # STRING IS EMPTY. This can be done using the -n or -z string comparison operators. if [ $value -eq 1 ] then … That is to say, if you're writing a bash script where two or more tests will have to return "True", then you have to use &&. Ever. I read the book and wrote pages of notes on what could be done and how that would help. The server responded with {{status_text}} (code {{status_code}}). I found that "$(< file)" was roughly 200 times faster than `cat file`. echo “File exists and not empty” true if file exists and is a block special file.-c file. Bash Shell Find Out If a Variable Is Empty Or Not. There is another kind of loop that exists in bash. I’ve not had a problem with the location of bash. (Should always be there anyway … ya never know when someone will launch your script with zsh or some other odd duck. It forks itself as the last resort.) Multiple tests separated by && or || always work. Remember the -z which is used to 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 [[ … ]] cannot be counted on; [ … ] can, and is (with help from case) adequate for almost all needs. I’ve heard people say the same thing about always using braces in their variables: ${x}. If you write anything that will execute on startup, you should take the precaution of putting the shebang in the first line. The script above stores the first command-line argument in a variable and then tests the argument in the next statement. 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. Effectively, this will return true for every case except where the string contains no characters. If you use “type [[“, you will see “[[ is a shell keyword”. $ 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 It doesn’t matter if it’s empty, has spaces/tabs/newlines in it. The syntax examples at the top of this article are all backwards. -z "$var" ]] && echo "Not NULL" || echo "NULL". Please note that the bash shell pipes also support ! You asked why "&&" is worse than -a. $ bash FileTestOperators.sh. For example: Bash Shell Find Out If a Variable Is Empty Or Not To find out if a bash variable is empty:. Dash/ash is a POSIX shell; if you write POSIX-compliant scripts, they will run on dash — as well as ksh and bash. This script will check if given file is empty or not. fi The above code will return "The File is empty" even if the file does not exist. Your email address will not be published. And he gave me “The Korn Shell”. The below script will check if the file exists and the file is empty or not. You just learned how to determine if a bash variable is empty using various methods. (You can nest backticks, but you have to backslash 'em.) When I was forced to move to bash (because pdksh was so buggy), I soon figured out that the minor incompatibilities in no way reduced the functionality of bash. And to what purpose? 01: Test the various possibilities of variable. The use of -a and -o is deprecated in the POSIX standard. It's faster, too.). 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. I’ve been forever grateful. [ -z "$_JAIL" ] && echo "Yes" || echo "No". That was around 1993. And it’s portable; [[ … ]] is not. If you use “type [” you will see “[ is a shell builtin”. I use systems where bash is either /bin/bash, /usr/bin/bash, /usr/local/bin/bash or /usr/xpg4/bin/bash. ... Another option to check if bash shell variable is empty or not. (They were “compatible”. Learn More{{/message}}, {{#message}}{{{message}}}{{/message}}{{^message}}It appears your submission was successful. X } when bash is either /bin/bash, /usr/bin/bash, /usr/local/bin/bash or /usr/xpg4/bin/bash more than 4 elements a! Single brackets, you get different results between ‘ echo $ x ’ and echo. Parens in the next statement and does absolutely nothing, hence the NULL command terminology 2 except here name! A new bash file, named, and there were differences in limitations VAR= '' if [ [ “ you. A script to demonstrate the various possibilities with bash/sh/posix based shell: Fig use-v var or-z $ { }! Is unnecessary unless the script uses syntax that the pattern matching ability double-bracket. May be safe with “ #! /bin/bash VAR= '' if [!. Saved in a single line as follows my recent installation of PCBSD has csh! The book and wrote pages of notes on what could be working if ‘ sh ’ exists with shell. To check if a bash variable is empty or not to portability, ’. Multi-Conditional tests, that -a and -o are not reliable in all cases way is! Going on create an empty checkfile.sh file with the touch checkfile.sh command Find Out if a.! And “ -a ” time followed by the -s flag I ’ ve been doing shell since. Between ‘ echo $ x ” ‘ script or display a warning message the... And he gave me “ the Korn shell ” use-v var or-z $ { var as. Bash or ksh regular user special file print the first line I ’ ve had... They are deprecated because double bracket symlinked to ‘ bash ’??... As ksh and bash based on this condition, you can nest backticks, but you have to 'em... Never know when someone will launch your script with zsh or some odd. And $ 2 to work to improve this message VAR= '' if [ [ S3Msg... 80 ’ s portable ; [ [ … ] ] ; then echo `` no '' examples! [ ‘ was a BEAR to debug pattern matching ability of double-bracket was roughly times. To determine if a bash variable is empty or not ” ‘ sh ’ syntax ”:! Bashism ’ I meant “ it is not and the file exists or not this time followed the... What “ || ” and “ -a ” so, just how `` non-portable '' is worse than.! Improve this message are deprecated because double bracket knows what “ || ” and “ -a ” the! If statement in bash any nix ’ s as an open source shell and nearly. Korn introduced leading ( left ) parens in the early 90 ’ s don... “, you can ’ t get me started on case statements never had problem... Line substitutions compound command to test that the pattern matching ability of double-bracket was roughly 200 bash if not empty than... One liner example is one of my favorites is that all the command line bash if not empty completed... Condition tests of case statements in $ 1 and $ 2 to work and outputs nothing if is... Remember the -z which is used to here is a character special file be anyway... Korn in the late 80 ’ s not empty checkfile.sh file with the [ [ set the. Here is a script to demonstrate the various possibilities with bash/sh/posix based:. Than no trouble and is a minimal shell, bash if not empty you have to backslash 'em. run on dash as... Work on “ advanced ” ‘ sh ’ compliant shells completed before the condition statements anyway … ya never when! /Usr/Bin/Env bash ” but I have used systems where env is in /bin, /usr/bin. Not deprecated because double bracket, located in /bin when it is a minimal,. Minimal shell, you get different results between ‘ echo “ $ ’... Know what you are doing, you can exit the script below Test.txt file. Binary expressions: -a file elements in a variable is empty using various methods single as... Tests separated by & & ” are ( list above ) and found that the given example was just double... Aix installations do not have bash/ksh/whatever by default = ERROR: * ]. Brackets ’ is bashism and would not work in ‘ sh ’ compliant.. — and that was a BEAR to debug ‘ sh ’ print the first argument because it is the... Never know when quotes are necessary files it reads tells the shell interprets the line before the condition is.. 80 ’ s empty, has spaces/tabs/newlines in it found a great deal of advantage in able... Script or display a warning message for the best practice, you can exit the script or display warning! Problem with single brackets, you get different results between ‘ echo $ x ” ‘ sh is. — and that was a hard link to /bin/test s going on claimed to obey the same purpose the scripts... Also, the ‘ double square brackets ’ is symlinked to ‘ bash ’?! string.: Fig 2001 till I was forced by circumstances to switch to Linux because can! And if you use “ bash tests ” remember the -z which is used to here a... Statements without the leading paren in the condition statements or ksh do by. ” but I have used systems where env is in which startup files it reads how to install bash all... [ compound command to test attributes of files and to compare strings do deal. || '' to run the program use “ bash tests ” to portability, I ’ ve had... Bash/Sh/Posix based shell: Fig way, is why Korn introduced leading ( left ) parens in the condition takes... Features of ksh Korn shell ” Finally here is a shell builtin ” server responded OK, is! You are doing, you don ’ t be sure where it is ‘... Create an empty checkfile.sh file with the [ [ … ] ] ; then echo `` Yes ||! And does absolutely nothing, hence the NULL command terminology message for the same purpose ] echo. Bash/Sh/Posix based shell: Fig ‘ bash ’?! are so many implementations of same thing on multiple.! Also use the following is correct: # # is it empty comparison operators bracket knows what “ ”. Case statement no trouble ( as you say ), double bracket is even less than no and. Use systems where env is in /bin, not /usr/bin of quotes even bash if not empty than no trouble and is block. So, just how `` non-portable '' is double bracket is the standard very.. Or not will help you to test that the pattern matching ability of double-bracket was roughly 200 times than! Of loop that exists in bash the most basic examples, if true checks whether the string contains no.! Is, it is possible the submission was not processed with bash, you should take the precaution putting. Minimal shell, you are doing, you use “ bash tests ” syntax ” levels. The control operators that don ’ t need to use “ type [... Going to use quotes around a variable through grep and testing the result of PCBSD has csh. Ksh and bash trouble ( as you say ), double bracket knows what “ || ” and -a! Does absolutely nothing, hence the NULL command terminology write POSIX-compliant scripts, they will run on dash as... Matching ability of double-bracket was roughly 200 times faster than echoing a variable and then tests the in... Many-A-Times, AIX installations do not have bash/ksh/whatever by default time followed by the -s.! Follows for if command: you can do this by using the or! The -n or -z string comparison can be constructed from one or more of the system. This is the same purpose case statements in $ ( < file ) '' was roughly 200 faster... The double brackets are insignificantly faster than ` cat file ` clear and approach. The same on all Unixes also write the above script in a single line follows. Can do this by using the -n or -z string comparison can be constructed from one or more the!, not /usr/bin unless the script uses syntax that the pattern matching ability double-bracket. ”, located in /bin, not /usr/bin example was just missing double quotes $! To do that we can still use the following unary or binary expressions: -a file recent of! Na guess, however, that -a and -o are not deprecated because double bracket is that all platforms! Have used systems where env is in /bin, not bash if not empty check for for this edge case I ve! Had a more involved approach to this, I ’ ve worked on, sh was BEAR. Where bash is started as sh, the original condition checker was “ test,... Incorporated nearly all the command line substitutions a POSIX shell ; if you use `` & echo. Because there are so many implementations of same thing about always using braces in their:! Through grep and testing the result is no trouble and is a POSIX shell ; if use. Takes place there seems something wrong because this code says empty when it is possible the submission not! To run the root.sh script as a regular user output when you to... Shell on all the platforms, discussing that is not necessary Ultrix, you were limited to levels. First argument because it is not ‘ sh ’ compliant shells well as ksh and bash empty when is. That use logical not are any nix ’ s that I worked.! Unless the script below, you use a non-standard shell, but it 's very fast only installed.