Linux – What Is Exit Status and List of All Status Codes

bashexit-statuslinuxscriptingshell-script

I'm trying to learn Linux bash scripting and I'm read through the articles in the http://tldp.org sites I saw some kind log clearing scripting I notice that something is used for as an exit status.
I have given below a few script snippet from the article.

#!/bin/bash
# Cleanup, version 3

#  Warning:
#  -------
#  This script uses quite a number of features that will be explained
#+ later on.
#  By the time you've finished the first half of the book,
#+ there should be nothing mysterious about it.

LOG_DIR=/var/log
ROOT_UID=0     # Only users with $UID 0 have root privileges.
LINES=50       # Default number of lines saved.
E_XCD=86       # Can't change directory?
E_NOTROOT=87   # Non-root exit error.

What do the E_NOTROOT(86) and E_XCD(87), if both variables use the reserved
exit status code for the program or not?

If both variables just use the random number for this purpose.

Reference: http://tldp.org/LDP/abs/html/abs-guide.html

Best Answer

Every execution has an Exit status. In general, zero means OK and non-zero is an error. That value is not shown naturally in the standard output. You can see that value typing echo $? after every executed command.

For example if you type:

mkdir test;echo $? If you have the right perms, you will create the directory and then you will see a zero.

but if you write mkdir testing/test;echo $? having the right perms but without having the "testing" subdir, you will see the error message and then a "1".

This is a very important tool in GNU/Linux because the commands can be interconnected. And (just as example) if you use double-ampersand for connecting commands, the second command ONLY is executed if the first command has a zero value on the exit. There are many ways to connect commands. To learn more just type man bash

Related Solutions

Bash – Simultaneously check for empty output and successful exit status

First of all, while they are functionally equivalent, $(…) is widely considered to be clearer than `…` — see this, this, and this. Secondly, you don’t need to use $? to check whether a command succeeded or failed. My attention was recently drawn to Section 2.9.1, Simple Commands of The Open Group Base Specifications for Shell & Utilities (Issue 7):

A "simple command" is a sequence of optional variable assignments and redirections, in any sequence, optionally followed by words and redirections, terminated by a control operator.

When a given simple command is required to be executed …
⋮ (blah, blah, blah …)

If there is a command name, execution shall continue as described in Command Search and Execution. If there is no command name, but the command contained a command substitution, the command shall complete with the exit status of the last command substitution performed. …

For example,

the exit status of the command
```
ls -ld "module_$(uname).c"
```
is the exit status from the ls, but
the exit status of the command
```
myfile="module_$(uname).c"
```
is the exit status from the uname.

So ferada’s answer can be streamlined a bit:

if output=$(/etc/grub.d/30_os-prober)  &&  [ -z "$output" ]
    # i.e., if 30_os-prober successfully produced no output
then
    install_linux_only
else
    install_dual_boot
fi

Note that it is good practice to use all-upper-case names only for environment variables (or variables to be visible throughout the script). Variables of limited scope are usually named in lower case (or, if you prefer, camelCase).

Bash – Execute two commands and get exit status 1 if first fails

Save and reuse $?.

test; ret=$?; report; exit $ret

If you have multiple test commands and you want to run them all, but keep track of whether one has failed, you can use bash's ERR trap.

failures=0
trap 'failures=$((failures+1))' ERR
test1
test2
if ((failures == 0)); then
  echo "Success"
else
  echo "$failures failures"
  exit 1
fi

Best Answer

Related Solutions

Bash – Simultaneously check for empty output and successful exit status

Bash – Execute two commands and get exit status 1 if first fails

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