Scripting Methods & trimming output

*  Return value of the command

Each command returns its exit status (variable: "$?") as the return value.

Table 1.21. Command exit codes

command exit status	numeric return value	logical return value
success	zero, 0	TRUE
error	non-zero, -1	FALSE

For example, try the following.

$ [ 1 = 1 ] ; echo $?
0
$ [ 1 = 2 ] ; echo $?
1

	Note
	Please note that, in the logical context for the shell, success is treated as the logical TRUE which has 0 (zero) as its value. This is somewhat non-intuitive and needs to be reminded here.

Typical command sequences and shell redirection

Let's try to remember following shell command idioms typed in one line as a part of shell command.

Table 1.22. Shell command idioms

command idiom	description
command &	background execution of command in the subshell
command1 | command2	pipe the standard output of command1 to the standard input of command2 (concurrent execution)
command1 2>&1 | command2	pipe both standard output and standard error of command1 to the standard input of command2 (concurrent execution)
command1 ; command2	execute command1 and command2 sequentially
command1 && command2	execute command1; if successful, execute command2 sequentially (return success if both command1 and command2 are successful)
command1 || command2	execute command1; if not successful, execute command2 sequentially (return success if command1 or command2 are successful)
command > foo	redirect standard output of command to a file foo (overwrite)
command 2> foo	redirect standard error of command to a file foo (overwrite)
command >> foo	redirect standard output of command to a file foo (append)
command 2>> foo	redirect standard error of command to a file foo (append)
command > foo 2>&1	redirect both standard output and standard error of command to a file "foo"
command < foo	redirect standard input of command to a file foo
command << delimiter	redirect standard input of command to the following lines until "delimiter" is met (here document)
command <<- delimiter	redirect standard input of command to the following lines until "delimiter" is met (here document, the leading tab characters are stripped from input lines)

The Debian system is a multi-tasking system. Background jobs allow users to run multiple programs in a single shell. The management of the background process involves the shell builtins: jobs, fg, bg, and kill. Please read sections of bash(1) under "SIGNALS", and "JOB CONTROL", and builtins(1).
For example, try the following

$ </etc/motd pager

$ pager </etc/motd

$ pager /etc/motd

$ cat /etc/motd | pager

Although all 4 examples of shell redirections display the same thing, the last example runs an extra cat command and wastes resources with no reason.
The shell allows you to open files using the exec builtin with an arbitrary file descriptor.

$ echo Hello >foo
$ exec 3<foo 4>bar  # open files
$ cat <&3 >&4       # redirect stdin to 3, stdout to 4
$ exec 3<&- 4>&-    # close files
$ cat bar
Hello

Here, "n<&-" and "n>&-" mean to close the file descriptor "n".
The file descriptor 0-2 are predefined.

Table 1.23. Predefined file descriptors

device	description	file descriptor
stdin	standard input	0
stdout	standard output	1
stderr	standard error	2

Command alias

You can set an alias for the frequently used command.
For example, try the following

$ alias la='ls -la'

Now, "la" works as a short hand for "ls -la" which lists all files in the long listing format.
You can list any existing aliases by alias (see bash(1) under "SHELL BUILTIN COMMANDS").

$ alias
...
alias la='ls -la'

You can identity exact path or identity of the command by type (see bash(1) under "SHELL BUILTIN COMMANDS").
For example, try the following

$ type ls
ls is hashed (/bin/ls)
$ type la
la is aliased to ls -la
$ type echo
echo is a shell builtin
$ type file
file is /usr/bin/file

Here ls was recently searched while "file" was not, thus "ls" is "hashed", i.e., the shell has an internal record for the quick access to the location of the "ls" command.