Bash operators
You have a shell script and wonder what these operators do? Then this article is for you.
Contents
* and ?
You may already know these used for so called "shell globbing" (pattern matching). * replaces an arbitrary amount of characters (including none), while ? replaces exactly one character.
These wildcards are actually bash operators! Imagine, we have the files a1, a2:
cp a*
now is a valid bash command which overrides one of these files with the other one.
$()
The operator $() in the bash shell is replaced by the output of the command enclosed in the parentheses. It is equivalent to backticks (``), but can be cascaded more easily.
Examples
rpm -ql $(rpm -qa)
for i in $(seq 1 1 100); do echo $i; done
$(())
The operator $(()) in the bash shell is replaced by the arithmetic result of the expression enclosed in the parentheses.
Examples:
# echo $((2*2)) 4
$$
The operator $$ delivers the id of the currently-running process.
Example:
# kill -9 $$
kills the current process
$!
The operator $! delivers the id of the process that has most recently sent to the background.
Example:
# xosview & sleep 5; kill $!
shows xosview for 5 seconds and stops it
> and >>
> and >> can be used to redirect the output (only stdout, not stderr, for that see below) of a command to a file. The difference between them is, that >> appends to the given files, while > will truncate it.
Examples:
ls -alh > files-in-directory.txt ls -alh /otherdir >> files-in-directory.txt # Append list of files in other directory
2> or 2>>
stderr can be redirected by using 2> or 2>>.
But sometimes, you need stderr to be included in stdout for certain purpose, e.g. when another program is parsing/logging only stdout and you need errors to be in there, too. In these cases, you can redirect stderr to stdout with "2>&1". (Of course, it is also possible to redirect stdout to stderr this way, if you need to silence a program on stdout for some purpose).
Examples:
cat notexisting 2> test # Writes "File or directory not found." to test cat notexisting 2>&1 # Outputs "File or directory not found." to stdout
|
| is used to pipe the stdout of one programm to the stdin of another one.
tail -n 100 foo | sort -r # Reverse sort the last 100 lines of the file foo
&&
&& evaluates the binary result of the command left of it and ands it with the binary result right of it if it is not already false, where false!=0. In other words, if you write
command1 && command2
command2 will only be executed if command1 returned success.
