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-rw-r--r--bash.html.markdown139
1 files changed, 113 insertions, 26 deletions
diff --git a/bash.html.markdown b/bash.html.markdown
index 14366e4c..8c40931e 100644
--- a/bash.html.markdown
+++ b/bash.html.markdown
@@ -15,6 +15,8 @@ contributors:
- ["Leo Rudberg", "https://github.com/LOZORD"]
- ["Betsy Lorton", "https://github.com/schbetsy"]
- ["John Detter", "https://github.com/jdetter"]
+ - ["Harry Mumford-Turner", "https://github.com/harrymt"]
+ - ["Martin Nicholson", "https://github.com/mn113"]
filename: LearnBash.sh
---
@@ -24,65 +26,92 @@ Nearly all examples below can be a part of a shell script or executed directly i
[Read more here.](http://www.gnu.org/software/bash/manual/bashref.html)
```bash
-#!/bin/bash
+#!/usr/bin/env bash
# First line of the script is shebang which tells the system how to execute
# the script: http://en.wikipedia.org/wiki/Shebang_(Unix)
# As you already figured, comments start with #. Shebang is also a comment.
# Simple hello world example:
-echo Hello world!
+echo Hello world! # => Hello world!
# Each command starts on a new line, or after semicolon:
echo 'This is the first line'; echo 'This is the second line'
+# => This is the first line
+# => This is the second line
# Declaring a variable looks like this:
Variable="Some string"
# But not like this:
-Variable = "Some string"
+Variable = "Some string" # => returns error "Variable: command not found"
# Bash will decide that Variable is a command it must execute and give an error
# because it can't be found.
# Or like this:
-Variable= 'Some string'
+Variable= 'Some string' # => returns error: "Some string: command not found"
# Bash will decide that 'Some string' is a command it must execute and give an
# error because it can't be found. (In this case the 'Variable=' part is seen
# as a variable assignment valid only for the scope of the 'Some string'
# command.)
# Using the variable:
-echo $Variable
-echo "$Variable"
-echo '$Variable'
+echo $Variable # => Some string
+echo "$Variable" # => Some string
+echo '$Variable' # => $Variable
# When you use the variable itself — assign it, export it, or else — you write
# its name without $. If you want to use the variable's value, you should use $.
# Note that ' (single quote) won't expand the variables!
# Parameter expansion ${ }:
-echo ${Variable}
+echo ${Variable} # => Some string
# This is a simple usage of parameter expansion
# Parameter Expansion gets a value from a variable. It "expands" or prints the value
# During the expansion time the value or parameter are able to be modified
# Below are other modifications that add onto this expansion
# String substitution in variables
-echo ${Variable/Some/A}
+echo ${Variable/Some/A} # => A string
# This will substitute the first occurrence of "Some" with "A"
# Substring from a variable
Length=7
-echo ${Variable:0:Length}
+echo ${Variable:0:Length} # => Some st
# This will return only the first 7 characters of the value
+echo ${Variable: -5} # => tring
+# This will return the last 5 characters (note the space before -5)
+
+# String length
+echo ${#Variable} # => 11
# Default value for variable
-echo ${Foo:-"DefaultValueIfFooIsMissingOrEmpty"}
+echo ${Foo:-"DefaultValueIfFooIsMissingOrEmpty"}
+# => DefaultValueIfFooIsMissingOrEmpty
# This works for null (Foo=) and empty string (Foo=""); zero (Foo=0) returns 0.
# Note that it only returns default value and doesn't change variable value.
+# Declare an array with 6 elements
+array0=(one two three four five six)
+# Print first element
+echo $array0 # => "one"
+# Print first element
+echo ${array0[0]} # => "one"
+# Print all elements
+echo ${array0[@]} # => "one two three four five six"
+# Print number of elements
+echo ${#array0[@]} # => "6"
+# Print number of characters in third element
+echo ${#array0[2]} # => "5"
+# Print 2 elements starting from forth
+echo ${array0[@]:3:2} # => "four five"
+# Print all elements. Each of them on new line.
+for i in "${array0[@]}"; do
+ echo "$i"
+done
+
# Brace Expansion { }
# Used to generate arbitrary strings
-echo {1..10}
-echo {a..z}
+echo {1..10} # => 1 2 3 4 5 6 7 8 9 10
+echo {a..z} # => a b c d e f g h i j k l m n o p q r s t u v w x y z
# This will output the range from the start value to the end value
# Builtin variables:
@@ -121,6 +150,7 @@ then
else
echo "Your name is your username"
fi
+# True if the value of $Name is not equal to the current user's login username
# NOTE: if $Name is empty, bash sees the above condition as:
if [ != $USER ]
@@ -133,7 +163,11 @@ if [ "" != $USER ] ...
# There is also conditional execution
echo "Always executed" || echo "Only executed if first command fails"
+# => Always executed
echo "Always executed" && echo "Only executed if first command does NOT fail"
+# => Always executed
+# => Only executed if first command does NOT fail
+
# To use && and || with if statements, you need multiple pairs of square brackets:
if [ "$Name" == "Steve" ] && [ "$Age" -eq 15 ]
@@ -146,13 +180,30 @@ then
echo "This will run if $Name is Daniya OR Zach."
fi
+# There is also the =~ operator, which tests a string against a Regex pattern:
+Email=me@example.com
+if [[ "$Email" =~ [a-z]+@[a-z]{2,}\.(com|net|org) ]]
+then
+ echo "Valid email!"
+fi
+# Note that =~ only works within double [[ ]] square brackets,
+# which are subtly different from single [ ].
+# See http://www.gnu.org/software/bash/manual/bashref.html#Conditional-Constructs for more on this.
+
+# Redefine command 'ping' as alias to send only 5 packets
+alias ping='ping -c 5'
+# Escape alias and use command with this name instead
+\ping 192.168.1.1
+# Print all aliases
+alias -p
+
# Expressions are denoted with the following format:
-echo $(( 10 + 5 ))
+echo $(( 10 + 5 )) # => 15
# Unlike other programming languages, bash is a shell so it works in the context
# of a current directory. You can list files and directories in the current
# directory with the ls command:
-ls
+ls # Lists the files and subdirectories contained in the current directory
# These commands have options that control their execution:
ls -l # Lists every file and directory on a separate line
@@ -169,7 +220,10 @@ cat file.txt
# We can also read the file using `cat`:
Contents=$(cat file.txt)
-echo "START OF FILE\n$Contents\nEND OF FILE"
+echo "START OF FILE\n$Contents\nEND OF FILE" # "\n" prints a new line character
+# => START OF FILE
+# => [contents of file.txt]
+# => END OF FILE
# Use `cp` to copy files or directories from one place to another.
# `cp` creates NEW versions of the sources,
@@ -190,10 +244,13 @@ mv s0urc3.txt dst.txt # sorry, l33t hackers...
# Since bash works in the context of a current directory, you might want to
# run your command in some other directory. We have cd for changing location:
cd ~ # change to home directory
+cd # also goes to home directory
cd .. # go up one directory
# (^^say, from /home/username/Downloads to /home/username)
cd /home/username/Documents # change to specified directory
cd ~/Documents/.. # still in home directory..isn't it??
+cd - # change to last directory
+# => /home/username/Documents
# Use subshells to work across directories
(echo "First, I'm here: $PWD") && (cd someDir; echo "Then, I'm here: $PWD")
@@ -203,6 +260,8 @@ pwd # still in first directory
mkdir myNewDir
# The `-p` flag causes new intermediate directories to be created as necessary.
mkdir -p myNewDir/with/intermediate/directories
+# if the intermediate directories didn't already exist, running the above
+# command without the `-p` flag would return an error
# You can redirect command input and output (stdin, stdout, and stderr).
# Read from stdin until ^EOF$ and overwrite hello.py with the lines
@@ -216,13 +275,15 @@ print("#stderr", file=sys.stderr)
for line in sys.stdin:
print(line, file=sys.stdout)
EOF
-
-# Run hello.py with various stdin, stdout, and stderr redirections:
-python hello.py < "input.in"
-python hello.py > "output.out"
-python hello.py 2> "error.err"
-python hello.py > "output-and-error.log" 2>&1
-python hello.py > /dev/null 2>&1
+# Variables will be expanded if the first "EOF" is not quoted
+
+# Run the hello.py Python script with various stdin, stdout, and
+# stderr redirections:
+python hello.py < "input.in" # pass input.in as input to the script
+python hello.py > "output.out" # redirect output from the script to output.out
+python hello.py 2> "error.err" # redirect error output to error.err
+python hello.py > "output-and-error.log" 2>&1 # redirect both output and errors to output-and-error.log
+python hello.py > /dev/null 2>&1 # redirect all output and errors to the black hole, /dev/null, i.e., no output
# The output error will overwrite the file if it exists,
# if you want to append instead, use ">>":
python hello.py >> "output.out" 2>> "error.err"
@@ -269,12 +330,19 @@ for Variable in {1..3}
do
echo "$Variable"
done
+# => 1
+# => 2
+# => 3
+
# Or write it the "traditional for loop" way:
for ((a=1; a <= 3; a++))
do
echo $a
done
+# => 1
+# => 2
+# => 3
# They can also be used to act on files..
# This will run the command 'cat' on file1 and file2
@@ -296,6 +364,7 @@ do
echo "loop body here..."
break
done
+# => loop body here...
# You can also define functions
# Definition:
@@ -306,6 +375,11 @@ function foo ()
echo "This is a function"
return 0
}
+# Call the function `foo` with two arguments, arg1 and arg2:
+foo arg1 arg2
+# => Arguments work just like script arguments: arg1 arg2
+# => And: arg1 arg2...
+# => This is a function
# or simply
bar ()
@@ -313,6 +387,8 @@ bar ()
echo "Another way to declare functions!"
return 0
}
+# Call the function `bar` with no arguments:
+bar # => Another way to declare functions!
# Calling your function
foo "My name is" $Name
@@ -320,25 +396,35 @@ foo "My name is" $Name
# There are a lot of useful commands you should learn:
# prints last 10 lines of file.txt
tail -n 10 file.txt
+
# prints first 10 lines of file.txt
head -n 10 file.txt
+
# sort file.txt's lines
sort file.txt
+
# report or omit repeated lines, with -d it reports them
uniq -d file.txt
+
# prints only the first column before the ',' character
cut -d ',' -f 1 file.txt
-# replaces every occurrence of 'okay' with 'great' in file.txt, (regex compatible)
+
+# replaces every occurrence of 'okay' with 'great' in file.txt
+# (regex compatible)
sed -i 's/okay/great/g' file.txt
+
# print to stdout all lines of file.txt which match some regex
# The example prints lines which begin with "foo" and end in "bar"
grep "^foo.*bar$" file.txt
+
# pass the option "-c" to instead print the number of lines matching the regex
grep -c "^foo.*bar$" file.txt
+
# Other useful options are:
grep -r "^foo.*bar$" someDir/ # recursively `grep`
grep -n "^foo.*bar$" file.txt # give line numbers
grep -rI "^foo.*bar$" someDir/ # recursively `grep`, but ignore binary files
+
# perform the same initial search, but filter out the lines containing "baz"
grep "^foo.*bar$" file.txt | grep -v "baz"
@@ -346,8 +432,9 @@ grep "^foo.*bar$" file.txt | grep -v "baz"
# and not the regex, use fgrep (or grep -F)
fgrep "foobar" file.txt
-# trap command allows you to execute a command when a signal is received by your script.
-# Here trap command will execute rm if any one of the three listed signals is received.
+# The trap command allows you to execute a command whenever your script
+# receives a signal. Here, trap will execute `rm` if it receives any of the
+# three listed signals.
trap "rm $TEMP_FILE; exit" SIGHUP SIGINT SIGTERM
# `sudo` is used to perform commands as the superuser