Questions about shell scripts, executable files that are interpreted by a shell (bash, zsh, etc.).

learn more… | top users | synonyms (2)

20
votes
1answer
4k views

Execute bash scripts on entering a directory

What is the best way to execute a script when entering into a directory? When I move into a new directory I would like bash to execute the projectSettings.bash script much like RVM does.
20
votes
2answers
801 views

Why escape trivial characters in shell script?

I just opened a legacy shell script (written in old ksh88 on Solaris) and found the following repeated all throughout the code: [ -f $myfile ] && \rm -f $myfile The escaping backslash ...
19
votes
3answers
954 views

What is the difference between [[ $a == z* ]] and [ $a == z* ]?

Is there is any difference between these two. [[ $a == z* ]] and [ $a == z* ] Can I have an example where they would have different outputs? Furthermore, how does the working of [[ ]] differs ...
19
votes
6answers
29k views

How can I test if a variable is empty or contains only spaces?

The following bash syntax verifies if param isn't empty: [[ ! -z $param ]] For example: param="" [[ ! -z $param ]] && echo "I am not zero" No output and its fine. But when param ...
19
votes
5answers
32k views

How to introduce timeout for shell scripting?

I want to run a shell script that got a loop in it and it can go for ever which I do not want to happen. So I need to introduce a timeout for the whole script. How can I introduce a timeout for the ...
19
votes
5answers
7k views

How can I get bash to exit on backtick failure in a similar way to pipefail?

So I like to harden my bash scripts wherever I can (and when not able to delegate to a language like Python/Ruby) to ensure errors do not go uncaught. In that vein I have a strict.sh, which contains ...
18
votes
3answers
1k views

Newlines in filenames

I understand and accept the premise that defensive1 shell scripting is both prudent and, in the longer term, more sustainable. Many of the answers to text processing questions here follow this ...
18
votes
2answers
43k views

How do I delete the first n lines of an ascii file using shell commands?

I have multiple files that contain ascii text information in the first 5-10 lines, followed by well-tabulated matrix information. In a shell script, I want to remove these first few lines of text so ...
18
votes
2answers
7k views

Are there naming conventions for variables in shell scripts?

Most languages have naming conventions for variables, the most common style I see in shell scripts is MY_VARIABLE=foo. Is this the convention or is it only for global variables? What about variables ...
18
votes
1answer
958 views

Is there a standard command that always exits with a failure?

I want to test my script with a command that fails. I could use an existing command with bad arguments. I could also write a simple script that immediately exits with a failure. Both of these are easy ...
17
votes
4answers
2k views

How Does Linux deal with shell scripts?

For this question, let's consider a bash shell script, though this question must be applicable to all types of shell script. When someone executes a shell script, does Linux load all the script at ...
17
votes
5answers
31k views

File extensions for unix shell scripts

On wikipedia, the article for .sh says: For the .sh file extension type, see Bourne shell. How about other unix shells? I know that the shebang is used inside the file to indicate an ...
17
votes
5answers
23k views

Passing named arguments to shell scripts

Is there any easy way to pass (receive) named parameters to a shell script? For example, my_script -p_out '/some/path' -arg_1 '5' And inside my_script.sh receive them as: # I believe this ...
17
votes
2answers
2k views

How did this '&' at the end of my command, make the script SO fast?

While solving some CTF challenges online, I came across a situation where I needed to bruteforce a server. This is the code I wrote: #!/bin/bash for i in {0..9}{0..9}{0..9}{0..9} do echo ...
17
votes
4answers
55k views

Splitting string by the first occurrence of a delimiter

I have a string in the next format id;some text here with possible ; inside and want to split it to 2 strings by first occurrence of the ;. So, it should be: id and some text here with possible ; ...
17
votes
6answers
11k views

Best way to follow a log and execute a command when some text appears in the log

I have a server log that outputs a specific line of text into its log file when the server is up. I want to execute a command once the server is up, and hence do something like the following: tail -f ...
17
votes
3answers
686 views

Finding the correct tmp dir on multiple platforms

I have a script that needs to create temporary files for its work, and clean up after itself. My question is about finding the right base directory for the temporary files. The script needs to work ...
17
votes
1answer
5k views

Can bash case statements cascade?

I'm trying to do something like this: case $level in 3) echo "Level Three" 2) echo "Level Two" 1) echo "Level one" ;; esac where if $level = 3, it ...
17
votes
2answers
310 views

Shell function definition: why is there a space after the opening brace?

You probably know of that fork bomb: :(){ :|:&};: #WARNING: harmful code I wonder why it is necessary, for it to parse, to include a space after the opening curly brace.
17
votes
3answers
30k views

Testing if a variable is empty in a shell script

I have seen the following technique used many times on many different shells, to test if a variable is empty: if [ "x$1" = "x" ]; then # Variable is empty fi Are there any advantages on using ...
16
votes
5answers
11k views

Bash script to get ASCII values for alphabet

How do I get the ASCII value of the alphabet? For example, 97 for a?
16
votes
8answers
3k views

What is the utility of the : command in shell scripting, given that it explicitly does nothing?

In the answer to this question about comments in shell scripting, it is indicated that the : is a null command that explicitly does nothing (but is not to be used for comments). What would be the ...
16
votes
7answers
4k views

How to efficiently generate large, uniformly distributed, random integers in bash?

I have been wondering what would be the best way to get good randomness in bash, i.e., what would be a procedure to get a random positive integer between MIN and MAX such that The range can be ...
16
votes
5answers
23k views

#!/bin/bash - no such file or directory

I've created a bash script but when I try to execute it, I get #!/bin/bash no such file or directory I need to run the command: bash script.sh for it to work. How can I fix this?
16
votes
3answers
19k views

Variable as command; eval vs bash -c

I was reading a bash script someone made and I noticed that the author doesn't use eval to evaluate a variable as a command The author used bash -c "$1" instead of eval "$1" I assume using eval ...
16
votes
3answers
8k views

How can one run multiple programs in the background with single command?

How can one run multiple programs in the background with single command? I have tried the commands below, but they do not work. nohup ./script1.sh & && nohup ./script2.sh & -bash: ...
16
votes
2answers
1k views

running script with “. ” and with “source ”

I was wondering if the following two ways of running a bash script are equivalent? . ./myScript.sh source myScript.sh Are they both running the content of the script instead of running the script, ...
16
votes
1answer
466 views

Why won't function return until background process ends?

Consider this script: #!/bin/bash function start { leafpad & echo $! } PID=$(start) echo "PID is $PID" The script does not continue past its closing brace until the leafpad process ends, ...
15
votes
4answers
16k views

Why doesn't “sudo su” in a shell script run the rest of the script as root?

A sample script can be as below: #!/bin/bash sudo su ls /root When using ./test.sh as the normal user, instead run ls as super user and exit, it switches to root; and when I logout, it executes ls ...
15
votes
4answers
6k views

Fix or alternative for mktemp in OS X

I'm looking at a bash script someone else wrote that uses mktemp: TEMP=`mktemp --directory` However, this line does not work on my machine (OS X 10.6). How would I fix this line so that it is ...
15
votes
6answers
2k views

Simultaneously calculate multiple digests (md5, sha256)?

Under the assumption that disk I/O and free RAM is a bottleneck (while CPU time is not the limitation), does a tool exist that can calculate multiple message digests at once? I am particularly ...
15
votes
5answers
10k views

How do I write a retry logic in script to keep retrying to run it upto 5 times?

I want to write logic in shell script which will retry it to run again after 15 sec upto 5 times based on "status code=FAIL" if it fails due to some issue.
15
votes
3answers
47k views

linux + add X days to date and get new virtual date

I have Linux ( RH 5.3) machine I need to add/calculate 10 days plus date so then I will get new date (expiration date)) for example # date Sun Sep 11 07:59:16 IST 2012 So I need to get ...
15
votes
3answers
4k views

Can't indent heredoc to match nesting's indent

If there's a "First World Problems" for scripting, this would be it. I have the following code in a script I'm updating: if [ $diffLines -eq 1 ]; then dateLastChanged=$(stat --format '%y' ...
15
votes
2answers
1k views

How does awk '!a[$0]++' work?

This one-liner removes duplicate lines from text input without pre-sorting. For example: $ cat >f q w e w r $ awk '!a[$0]++' <f q w e r $ The original code I have found on the internets ...
15
votes
5answers
695 views

Stateful bash function

I’d like to implement a function in Bash which increases (and returns) a count with every call. Unfortunately this seems non-trivial since I’m invoking the function inside a subshell and it ...
15
votes
9answers
47k views

How to compare two dates in a shell

How to compare two dates in a shell script? For example something like todate=2013-07-18 cond=2013-07-15 if [ $todate -ge $cond ]; then break fi It doesn't work. ...
15
votes
4answers
22k views

How can I use bash's if test and find commands together?

I have a directory with crash logs, and I'd like to use a conditional statement in a bash script based on a find command. The log files are stored in this format: /var/log/crashes/app-2012-08-28.log ...
15
votes
2answers
4k views

How do I test to see if an application exists in $PATH?

I'm trying to write all of my sh startup/env scripts to work with as much DRY and as much: "works on every *nix I clone it to", as possible. This means making sure that if I try to run code that's not ...
15
votes
2answers
5k views

Rule for invoking subshell in Bash?

I seem to misunderstand the Bash rule for creating a subshell. I thought parentheses always creates a subshell, which runs as its own process. However, this doesn't seem to be the case. In Code ...
14
votes
3answers
4k views

Determining if a file is a hard link or symbolic link?

I'm creating a shell script that would take a a filename/path to a file and determines if the file is a symbolic link or a hard link. The only thing is, I don't know how to see if they are a hard ...
14
votes
3answers
444 views

Prepare answers for questions of a command

Let's say I want to write a shell script that executes just one command. But this command is poorly designed. It doesn't offer any command line options; instead it asks some questions and waits for ...
14
votes
4answers
2k views

What is the meaning of IFS=$'\n' in bash scripting ?

at the beginning of a shell script program in the bash is the following line: IFS=$'\n' What is the meaning behind this collection of symbols ?
14
votes
10answers
43k views

Show sum of file sizes in directory listing

The Windows dir directory listing command has a line at the end showing the total amount of space taken up by the files listed. For example, dir *.exe shows all the .exe files in the current ...
14
votes
4answers
25k views

How to read from two input files using while loop

I wanted to know if there is any way of reading from two input files in a nested while loop one line at a time. For example, lets say I have two files FileA and FileB. FileA: [jaypal:~/Temp] cat ...
14
votes
3answers
25k views

Comparing two strings in Bash

I have the following if block in my bash script: if [ ${PACKAGENAME} -eq kakadu-v6_4-00902C ]; then echo "successfully entered if block!!" fi The script execution is not entering my if block even ...
14
votes
6answers
3k views

Portable way to get script's absolute path?

What is a portable way for a (zsh) script to determine its absolute path? On Linux I use something like mypath=$(readlink -f $0) ...but this is not portable. (E.g., readlink on darwin does not ...
14
votes
5answers
13k views

how do I trim each line in bash?

I would like to remove all leading and trailing spaces and tabs from each line in an output. Is there a simple tool like trim I could pipe my output into?
14
votes
5answers
3k views

Bash in Linux v.s Mac OS

I am buying an iMac. I've been working on Linux for a long time and I do a bunch of bash scripting. I was wondering if all the bash commands and features are available on Mac OS ? Anyone has any ...
14
votes
2answers
7k views

Simplest way to comment/uncomment certain lines using command line

Is there a way to comment/uncomment a shell/config/ruby script using command line? for example: $ comment 14-18 bla.conf $ uncomment 14-18 bla.conf this would add or remove # sign on bla.conf on ...