Linux - Bash

 Posted on January 2, 2022  | 

Running ways

Make the shell script executable
The script should begin with the shebang (e.g. #!/bin/bash).
Treat the shell script as the argument of sh
In this case, the shebang is not necessary.

Print

echo

  • By default, a line break is appended every time it is invoked.
  • The default line break can be avoided by option -n.

printf

  • Syntax
printf "format_list" content/variable_list

  • Format

    • %s

      • %3s: 3-character wide and right alignment.
      • %-3s: 3-character wide and left alignment.
    • %c
    • \n: line break.

    • %f

      • %5.2f: 5-character wide with 2 of them for decimal portion, right alignment.
      • %-5.2f: 5-character wide with 2 of them for decimal portion, left alignment.

Variable

In Bash, every variable is a string.

Plain variable

Assignment

var_name=var_value

Environment variable

List

cat /proc/$PID/environ

where $PID is an integer and can be obtained by pgrep, e.g.

pgrep emacs

Assignment

env_var_name=value
export env_var_name
...
export env_var_name=value
  • $SHELL or $0
  • $UID

  • $? indicates the exit state of last command.

    • 0 means successfully executed
    • Others mean failure

Get the length of variable's value (string)

${#var_name}

Mathematical operation (e.g. let)

$ in front of variable name is not needed.

let var=var1+var2
...
let var++
let var+=1
let var=var+1
...
let var--
let var-=1
let var=var-1

Array

array_var=(0, 1, 2, 3, 6)
...
echo #{array_var[0]}

Alias

alias new_command='command sequence'
...
new_command

Function

function func_name()
{
statements
}
...
func_name()
{
statements
}
...
func_name arg_list
  • $0: script name.
  • $n: the n-th argument, n=1, 2, …
  • $@: "$1" "$2" "$3"
  • $*: "$1c$2c$3", where c is the 1st character of IFS.

Test & comparison

Arithmetic

[ $var -eq 0 ]
[ $var -ne 0 ]
[ $var -gt 0 ]
[ $var -ge 0 ]
[ $var -lt 0 ]
[ $var -le 0 ]
[ $var1 -ne 0 -a $var2 -lt 1 ]
[ $var1 -ne 0 -o $var2 -lt 1 ]

File system

[ -f $var ] # is $var a file?
[ -d $var ] # is $var a directory?
[ -x $var ] # is $var executable?
[ -e $var ] # does $var exist?
[ -w $var ] # is $var writable?
[ -r $var ] # is $var readable?
[ -L $var ] # is $var a symbolic link?
[ -b $var ] # is $var a block device?
[ -c $var ] # is $var a character device?

String

[[ -z $str ]] # is $str an empty string?
[[ -n $str ]] # is $str a non-empty string?
[[ $str1==$str2 ]]
[[ $str1!=$str2 ]]
[[ $str1>$str2 ]]
[[ $str1<$str2 ]]
[[ -n $str1 ]] && [[ -z $str2 ]]
[[ -n $str1 ]] || [[ -z $str2 ]]

Judgement

if condition; then
commands
fi
...
[ condition ] && command
...
[ condition ] || command
...
if condition; then
commands1
else
commands2
fi
...
if condition1; then
commands1
elif condition2; then
commands2
else
commands3
fi

Iteration

for var in list;
do
commands # use $var
done
...
while condition
do
commands # use $var
done
...
until condition
do
commands # use $var
done

Miscellaneous

  • Commands are seperated by semicolon (;) or line break.
  • File descriptor is an abstract pointer for file access.

  • Internal field separator (IFS)

    • $IFS is the environment variable to store IFS.
    • Default values: space, table, line break.
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