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Find the exit status of a previous command in Bash

Ryan — Fri, 14 Jan 2011 17:30:01 +0000

Most programs will return an exit status of 0 if the program was successful, while a non-zero exit status usually indicates an error. You can find the exit status or exit code of the previously executed command by accessing the “$?” shell variable.

$ COMMAND $ echo $?

Typically when a command terminates on a fatal signal whose number is N, Bash uses the value 128+N as the exit status. If a command is not found, the child process created to execute it returns a status of 127. If a command is found but is not executable, the return status is 126.

You can also use the exit status in a conditional.

#!/bin/bash COMMAND STATUS=$? if [ $STATUS -eq 0 ]; then echo "Command Successful" else echo "Command Failed" fi

Find the parent PID of a Bash Script

Ryan — Sun, 09 Jan 2011 17:53:03 +0000

For any number of reasons you may want to know the parent process ID of the current Bash script. You can find the parent process of the current Bash script or shell by printing the ‘$PPID‘ shell variable. $ echo $PPID20341 Related postsFind the PID of the current Bash script Find the PID of a [...] Related posts

Find the PID of a background child process in Bash

Ryan — Sun, 09 Jan 2011 17:47:57 +0000

In order to control or monitor background child processes from a shell script you will need to know the PID of the child. Bash stores the PID of the last process executed in the “$!” shell variable.

If you start a background process in an interactive shell it will output the PID of the child.
$ sleep 1 &[1]20450

Since thee PID of the last command set to run in the background by the current shell or script is stored in ‘$!‘ variable, you can simply echo this variable to print the PID of the previous sleep command.
$ echo $!20450

Find the PID of the current Bash script

Ryan — Sun, 09 Jan 2011 17:18:51 +0000

In many cases you will need to determine the PID of a current Bash script or shell. Bash stores a specific variable that allows you to view the process ID of the current shell “$$‘.

You can echo the $$ to print the current PID.
$ echo $$23490

Convert a relative path to absolute path in BASH

Ryan — Fri, 01 Oct 2010 15:11:41 +0000

There are a few ways to take a relative path and convert it to an absolute path which can be accessed regardless of the current working directory. The easiest method is to use the readlink utility which comes bundled with about every distribution.

Here my working directory is my home directory. It is printed as an absolute path.
$ pwd/home/ryan

Using readlink we can convert this same directory as a relative path to an absolute path.
$ readlink -f ./../ryan/Desktop/../../../home/ryan/home/ryan

If you are using a Debian based distribution like Ubuntu you can use the realpath utility instead.
$ realpath ./../ryan/Desktop/../../../home/ryan/home/ryan

Multiple line comments in BASH

Ryan — Wed, 29 Sep 2010 18:53:03 +0000

Comments are done in BASH and most other shells by placing a ‘#’ mark at the beginning of a line. To create a multi-line comment, or to comment out an entire block of code, you can use the following HERE DOCUMENT feature.

: << '--COMMENT--' comment line 1 comment line 2 comment line n --COMMENT--

Single versus double quotes in BASH

Ryan — Thu, 16 Oct 2008 03:50:30 +0000

Understanding the difference between double versus single quotes is important when using BASH. Many times you may have seen them being used interchangeably. The basic difference is that variable names will be expanded within double quotes but not within single ones.

This example shows the normal output with no quotes.

$ echo $USER

ryan

As you can see the shell will expand the variable.

In this example we will surround the variable with double quotes.

$ echo "$USER"

ryan

We see the same result as before.

This time with single quotes.

$ echo '$USER'

$USER

The variable name is not expanded.

Here is another interesting case. The single quotes are inside the double quotes.

$ echo "'$USER'"

'ryan'

The variable is expanded and the single quotes are retained.

Here I flip the order with the single quotes on the outside.

$ echo '"$USER"'

"$USER"

The variable is not expanded but the double quotes are retained.