About
This page is about Os Shell scripts (with a accent on the Bash shell)
How to create a shell script
A Bash or Shell Script is a text file that:
- has a shebang
- has the executable permission.
File Extension
File extensions are meaningless in UNIX, unlike DOS, where EXE, COM, and BAT indicate executable files.
Linux uses a shebang to define the type of language.
Example of minimal bash text file
#!/bin/bash
Executable
The .sh extension denotes shell script files but doesn't make the script executable. See file permission
Management
Start a script
In another process
Unlike DOS, UNIX does not automatically look in the current directory for a file to execute.
You have to specify:
- the full file name
/usr/local/scripts/myscript.sh
- or the dot (.) shortcut that refers to the current directory
cd /usr/local/scripts/
$ ./myscript.sh
Unix/Linux search for executables only in directories identified in the PATH variable.
In the same process
If you want to get the variable back, you need to run the script in the same process. For this, you use:
- or Bash - Dot
Run and terminate (exec)
Run a script in the background
- nohup is a POSIX command to ignore the HUP (hangup) signal, enabling the command to keep running after the user who issues the command has logged out.
- By adding the ampersand (&) to the end of the command line, you start the application in background.
Run a script hosted on the internet
bash <(curl -f -L -sS https://example.com/myscript.sh) args
Example:
bash <(curl -f -L -sS https://raw.githubusercontent.com/pagespeed/ngx_pagespeed/master/scripts/build_ngx_pagespeed.sh) --help
Exit Value
Bash’s exit status is the exit status of the last command executed in the file script. If no commands are executed, the exit status is 0.
Timing a script
$SECONDS: The number of seconds the script has been running.
#!/bin/bash
TIME_LIMIT=10
INTERVAL=1
echo
echo "Hit Control-C to exit before $TIME_LIMIT seconds."
echo
while [ "$SECONDS" -le "$TIME_LIMIT" ]
do # $SECONDS is an internal shell variable.
if [ "$SECONDS" -eq 1 ]
then
units=second
else
units=seconds
fi
echo "This script has been running $SECONDS $units."
# On a slow or overburdened machine, the script may skip a count
#+ every once in a while.
sleep $INTERVAL
done
echo -e "\a" # Beep!
exit 0
Arguments
Get the script Name
The special parameter 0 Expands to the name of the shell or shell script. This is set at shell initialization. If bash is invoked with a file of commands, $0 is set to the name of that file.
If bash is started with the -c option, then $0 is set to the first argument after the string to be executed, if one is present. Otherwise, it is set to the file name used to invoke bash, as given by argument zero.
# echo $0
/bin/bash
Get the script directory
- for a file
echo $(dirname $0)
# saw also
echo $( cd $(dirname $0) ; pwd -P )
- for a symlink
echo $( dirname $(realpath "$0") )
Absolute path name
See the underscore special parameter $_
# $() run a subshell therefore there the cd command has no effect
SCRIPT_PATH=$( cd $(dirname $0) ; pwd -P )
Checking syntax
With the -n or -o noexec option of the set command, the command are not executed and you can then check a shell script for syntax errors.
Configuration
$BASH_ENV
An environmental variable pointing to a Bash startup file to be read when a script is invoked