Bash Special Characters
=====
# Comments
comments can be occurred following the end of a command and even embedded with a pipe.
$ echo "test" |\
# delete `e` from it
ed -e 's/e//g'
tst
; semicolon
Command separator. It will permits putting two or more commands on the same line.
;; / ;;& / :&
Terminators of case option. ;;& / :& are compatible with verison 4+.
. dot
Equivalent to source, which is a bash built-in cmd.
, comma
links together a series of arithmetic operations. All are evaluated, but only the last one is returned.
$ let "b=((a=9, 15 / 3))"; echo $a $b
9 5
concatenate strings.
$ ls /{,tmp}
/:
bin boot dev etc export home lib lib64 media mnt opt proc root run sbin srv sys tmp usr var
/tmp:
bash_test.sh dido matrix pid set test test_pipe test.sh
convert to lowercase
$ name="MATRIX"; echo ${name,}; echo ${name,,}
mATRIX
matrix
\ backslash
Escape
``` backquote
command substitution
$ for i in `ls -1`; do echo $i; done
bash_test.sh
dido
matrix
pid
set
test
test_pipe
test.sh
: colon
null command. It will always return true and equivalent as NO-OP(do-nothing operation)
$ ls -1 | grep colon; echo $?
1
$ ls -1 | grep colon; :; echo $?
0
Provide a placeholder where a binary operation is expected.
$ n=1; $((n = $n + 1)); echo $n
-bash: 2: command not found
2
$ n=1; : $((n = $n + 1)); echo $n
2
Provide a placeholder where a command is expected.
$ n=1; if [ $n -eq 2 ]; then : ; else echo $n; fi
1
Used in parameter substitution / Variable expansion / substring replacement.
combination with the redirection operators
with >, truncates a file to zero length, without changing its permissions. If the file did not previously exist, creates it.
with >>, has no effect on a pre-existing target file. If the file did not previously exist, creates it.
NOTE: applies to regular files, not pipes, symlinks, and certain special files.
! exclamation
reverse
$ ! true; echo $?
1
$ true; echo $?
0
* asterisk
wild card for filename expansion or arithmetic operator
$ echo *
bash_test.sh dido matrix pid set test test_pipe test.sh
? question
test operator in format condition?result-if-true:result-if-false
$ n=1; (( m = n==1?2:3 )) ; echo $m
2
$’…’
Quoted string expansion
$ echo $'\ttest'
test
$ echo $'aa\x27'
aa'
() parentheses
List of commands within () starts a subshell
$ echo $BASH_SUBSHELL; (echo $BASH_SUBSHELL)
0
1
{a,b,c} brace
brace expansion
$ echo \"{These,words,are,quoted}\"
"These" "words" "are" "quoted"
{} curly brackets
anonymous function. unlike in a “standard” function, the variables inside it remain visible outside
$ { a=123; } ; echo $a
123
The code block enclosed in braces may have I/O redirected to and from it.
$ { echo a; echo b; } > /tmp/test ; cat /tmp/test
a
b
(())
interger expansion
$ a=$(( 2*2 )) ; echo $a
4
> / &> / >> / < / <>
redirection
&> redirect both stdout and stderr to file
<< redirect used in a here document which is a special-purpose code block. It uses a form of I/O redirection to feed a command list to an interactive program or a command, such as ftp, cat, etc.
$ cat ./test.sh
#!/bin/bash
sftp localhost <<EOF
cd /tmp
ls -1
quit
EOF
$ ./test.sh
Warning: Permanently added 'localhost' (ECDSA) to the list of known hosts.
Connected to localhost.
sftp> cd /tmp
sftp> ls -1
dido
matrix
subshell.sh
test
test.sh
test_pipe
sftp> quit
<<< redirect used in a here string which can be considered as a stripped-down form of a here document.
$ grep -q test <<< "This is a test string"; echo $?
0
same as
$ echo "This is a test string" | grep -q test ; echo $?
0
(command)> / <(command) process substitution
>| force overwrite
$ bash -C echo > test.sh
bash: test.sh: cannot overwrite existing file
$ ls -al test.sh; bash -C echo >| test.sh; ls -al test.sh
-rwxr-xr-x 1 jun_zou jun_zou 5 Aug 16 19:48 test.sh
/usr/bin/echo: /usr/bin/echo: cannot execute binary file
-rwxr-xr-x 1 jun_zou jun_zou 0 Aug 16 19:48 test.sh
- redirect from/to stdin or stdout
$ cat -
test
test
$ echo matrix | cat -
matrix
Redirect stdin
Transfer files to remote host with - redirect
$ ls -al /tmp/visual; tar cf - visual | ssh -q localhost 'tar xf - -C /tmp'; ls -al /tmp/visual
ls: cannot access /tmp/visual: No such file or directory
-rw-rw-r-- 1 jun_zou jun_zou 2845 Jul 28 2022 /tmp/visual
~+ current working directory
~- previous working directory
$ echo ~+; cd ~-; echo ~+ ; cd ~-; echo ~+
/tmp/test
/home/jun_zou
/tmp/test
$* / $@
The positional parameters starting from the first.
With doublequotes ", both are same. If without, $* will take all as a single word. $@ will take each parameter as a quoted string, that is, the parameters are passed on intact, without interpretation or expansion.
$ cat <<EOF> ./bash_special_characers.sh
#!/bin/bash
function print_all_parameters() {
echo "There are $# parameters here: "
i=1
while [ ${i} -le $# ]
do
echo "$i parameter: ${!i}"
: $((++i))
done
echo ""
}
echo "Unquoted \$*"
print_all_parameters $*
echo "Quoted \"\$*\""
print_all_parameters "$*"
echo "Unquoted \$@"
print_all_parameters $@
echo "Quoted \"\$@\""
print_all_parameters "$@"
EOF
$ ./bash_special_characers.sh 12 34
Unquoted $*
There are 2 parameters here:
1 parameter: 12
2 parameter: 34
Quoted $*
There are 1 parameters here:
1 parameter: 12 34
Unquoted $@
There are 2 parameters here:
1 parameter: 12
2 parameter: 34
Quoted $@
There are 2 parameters here:
1 parameter: 12
2 parameter: 34
When working with doublequotes ", all parameters in $* will put the first character of IFS between the elements in order to construct the final output string. But $& will not do in this way.
$ cat bash_special_parmeters.sh
#! /bin/bash
# bash_special_parmeters.sh - Cmd args - positional parameter demo
#### Set the IFS to | ####
IFS='|'
echo "Command-Line Arguments Demo"
echo "*** All args displayed using \$@ positional parameter ***"
echo "$@" #*** double quote added ***#
echo "*** All args displayed using \$* positional parameter ***"
echo "$*" #*** double quote added ***#
$ ./bash_special_parmeters.sh apple pear grape lemon
Command-Line Arguments Demo
*** All args displayed using $@ positional parameter ***
apple pear grape lemon
*** All args displayed using $* positional parameter ***
apple|pear|grape|lemon
$# hash mark
Number of positional parameters (decimal)
$ cat ./bash_special_parmeters.sh
#!/bin/bash
echo "$#"
$ ./bash_special_parmeters.sh 12 34
2
$?
Status of the most recently executed foreground-pipeline (exit/return code)
$- dash
Current option flags set by the shell itself.
$ cat bash_special_parmeters.sh
#!/bin/bash
echo "$-"
set -euo pipefail
echo "$-"
$ ./bash_special_parmeters.sh 12 34
hB
ehuB
$$
The process ID (PID) of the shell. In an explicit subshell it expands to the PID of the current “main shell”, not the subshell. This is different from $BASHPID!
$ echo $$; echo $BASHPID ; ( cd /usr; pstree -p | grep $$; echo "$$" )
33969
33969
| |-bash(33969)---bash(33868)-+-grep(33870)
33969
$ echo $$; echo $BASHPID ; ( cd /usr; pstree -p | grep $$; echo "$BASHPID" )
33969
33969
| |-bash(33969)---bash(34977)-+-grep(34979)
34977
$! exclamation mark
The process ID (PID) of the most recently executed background pipeline
$ ping -c 1000 localhost > /dev/null &
[1] 47589
$ echo $!
47589
$0 zero
The name of the shell or the shell script (filename).
$ echo $0
-bash
$ cat bash_special_parmeters.sh
#!/bin/bash
echo "$0"
$ ./bash_special_parmeters.sh
./bash_special_parmeters.sh
$_ underscore
A kind of catch-all parameter. Directly after shell invocation, it’s set to the filename used to invoke Bash, or the absolute or relative path to the script, just like $0 would show it. Subsequently, expands to the last argument to the previous command.
$ cat bash_special_parmeters.sh
#! /bin/bash
echo "$_"
$ ./bash_special_parmeters.sh
./bash_special_parmeters.sh
$ cat bash_special_parmeters.sh
#! /bin/bash
echo test
echo "$_"
$ ./bash_special_parmeters.sh
test
test