Shell Scripting – Set Command

In this article, we will see the SET command in bash scripting.

Set command: It is used to set or unset specific flags and settings( determines the behavior of the script and helps in executing the tasks without any issue.) inside the shell environment. It can be used to change or display the shell attributes and parameters. 

Syntax:

set -options arguments

The set command supports the following options:

Option	Description
-a	use to mark variables that are created or modified or created for export.
-b	use to notify the termination of the job.
-e	use to exit when the command exits with a non-zero status.
-f	it disables the file name generation known as globbing
-h	It saves the location of the command where it got looked.
-k	It places all assignment arguments in the environment variable of a command. Exception: It excludes all arguments that precede the command name.
-m	It enables job control
-n	It is used to read the commands.
-o	It is used for naming the option
-p	It disables the processing of the ‘$ENV’ file and also imports the shell functions. Turned on:  when the real and effective user ids do not match.
-t	It uses to exit from the command after executing a single command
-u	It treats unset variable as an error during substitution
-v	It prints the shell input lines
-x	It prints the commands and their arguments in the same sequence as they got executed.
-B	It performs the Brace expansion by the shell
-C	It disallows the existing regular files to be overwritten
-E	used when shell functions inherit the ERR trap
-H	It enables style history substitution. It is on by default.
-P	used when during command execution we don’t want to follow the symbolic links.
-T	set this flag, this helps shell functions to inherit the DEBUG trap

For a demonstration of the use of the set command, let’s use some set command.

Set -x command

This option prints the commands in the sequence as they got executed or is mainly used to do some script debugging..

Code:

set -x
echo Hello
echo Romy

Output:

bar
hello
Romy
+ echo bar
+ echo hello
+ echo Romy

We can see the commands getting printed after the command execution with the ‘+’ sign.

Set -e command

It terminates the execution when the error occurs.

Code:(without set -e)

echo Hello
foo
echo Romy

Output:

Hello
Romy
main.sh: line 14: foo: command not found

‘foo’ is a non-existent command but bash still executed the third line after encountering the error at the second line. We can use the set command to stop termination.

Code:(with set -e)

set -e
echo Hello
foo
echo Romy

Output:

Hello
main.sh: line 15: foo: command not found

We can see that the third line is not getting printed as the execution got terminated after the second line.

Piped commands

Set -e command does not work with piped commands.

Example:

set -e
foo | echo " This is the piped command"
echo "executed"

Output:

 This is the piped command
executed
main.sh: line 3: foo: command not found

We can see that third line is getting executed, instead of terminating the execution after the second line.

To overcome this problem, we have to use ‘set -eo pipefail’

Set -eo pipefail

Example:

set -eo pipefail

foo | echo " This is the piped command"

echo "executed"

Output:

 This is the piped command
main.sh: line 13: foo: command not found

Set positional parameters using set command

It can be used to assign values to positional parameters. Position of the value referenced as ${N} where N denotes the position of the parameter.

The $1 is the first positional parameter after the command. The $2 value is the second parameter, and so on.

Example:

set apple mango orange guava
echo $1
echo $2
echo $3
echo $4

Output:

apple
mango
orange
guava

Unset positional parameters

In order to unset the positional parameter, run the set command followed by two hyphens(set –).

Example:

set apple mango orange guava

set --
echo $1
echo $2

echo "Hello"

Output:

Hello

We can see that nothing is getting printed corresponding to the first two commands to print positional parameters.