BashControl Structures

Syntax

  • [ "$1" = "$2" ] #A "[" bracket is actually a command. Because of this it requires a space befor and after it.
  • test "$1" = "$2" #Test is a synonym for the "[" command

Parameters

Parameter to [ or testDetails
File OperatorsDetails
-e "$file"Returns true if the file exists.
-d "$file"Returns true if the file exists and is a directory
-f "$file"Returns true if the file exists and is a regular file
-h "$file"Returns true if the file exists and is a symbolic link
String ComparatorsDetails
-z "$str"True if length of string is zero
-n "$strTrue if length of string is non-zero
"$str" = "$str2"True if string $str is equal to string $str2. Not best for integers. It may work but will be inconsitent
"$str" != "$str2"True if the strings are not equal
Integer Comparators      Details
"$int1" -eq "$int2"True if the integers are equal
"$int1" -ne "$int2"True if the integers are not equals
"$int1" -gt "$int2"True if int1 is greater than int 2
"$int1" -ge "$int2"True if int1 is greater than or equal to int2
"$int1" -lt "$int2"True if int1 is less than int 2
"$int1" -le "$int2"True if int1 is less than or equal to int2

Remarks

There are many comparator parameters available in bash. Not all are yet listed here.

If statement

if [[ $1 -eq 1 ]]; then
    echo "1 was passed in the first parameter"
elif [[ $1 -gt 2 ]]; then
    echo "2 was not passed in the first parameter"
else
    echo "The first parameter was not 1 and is not more than 2."
fi

The closing fi is necessary, but the elif and/or the else clauses can be omitted.

The semicolons before then are standard syntax for combining two commands on a single line; they can be omitted only if then is moved to the next line.

It's important to understand that the brackets [[ are not part of the syntax, but are treated as a command; it is the exit code from this command that is being tested. Therefore, you must always include spaces around the brackets.

This also means that the result of any command can be tested. If the exit code from the command is a zero, the statement is considered true.

if grep "foo" bar.txt; then
    echo "foo was found"
else
    echo "foo was not found"
fi

Mathematical expressions, when placed inside double parentheses, also return 0 or 1 in the same way, and can also be tested:

if (( $1 + 5 > 91 )); then
    echo "$1 is greater than 86"
fi

You may also come across if statements with single brackets. These are defined in the POSIX standard and are guaranteed to work in all POSIX-compliant shells including Bash. The syntax is very similar to that in Bash:

if [ "$1" -eq 1 ]; then
    echo "1 was passed in the first parameter"
elif [ "$1" -gt 2 ]; then
    echo "2 was not passed in the first parameter"
else
    echo "The first parameter was not 1 and is not more than 2."
fi

While Loop

#! /bin/bash

i=0

while [ $i -lt 5 ] #While i is less than 5
do
    echo "i is currently $i"
    i=$[$i+1] #Not the lack of spaces around the brackets. This makes it a not a test expression
done #ends the loop

Watch that there are spaces around the brackets during the test (after the while statement). These spaces are necessary.

This loop outputs:

i is currently 0
i is currently 1
i is currently 2
i is currently 3
i is currently 4

For Loop

#! /bin/bash

for i in 1 "test" 3; do #Each space separated statement is assigned to i
    echo $i
done

Other commands can generate statements to loop over. See "Using For Loop to Iterate Over Numbers" example.

This outputs:

1
test
3

Using For Loop to List Iterate Over Numbers

#! /bin/bash

for i in {1..10}; do # {1..10} expands to "1 2 3 4 5 6 7 8 9 10"
    echo $i
done

This outputs the following:

1
2
3
4
5
6
7
8
8
10

For Loop with C-style syntax

The basic format of C-style for loop is:

for (( variable assignment; condition; iteration process ))

Notes:

  • The assignment of the variable inside C-style for loop can contain spaces unlike the usual assignment
  • Variables inside C-style for loop aren't preceded with $.

Example:

for (( i = 0; i < 10; i++ ))
do
    echo "The iteration number is $i"
done

Also we can process multiple variables inside C-style for loop:

for (( i = 0, j = 0; i < 10; i++, j = i * i ))
do
    echo "The square of $i is equal to $j"
done

Until Loop

Until loop executes until condition is true

i=5
until [[ i -eq 10 ]]; do #Checks if i=10
    echo "i=$i" #Print the value of i
    i=$((i+1)) #Increment i by 1
done

Output:

i=5
i=6
i=7
i=8
i=9

When i reaches 10 the condition in until loop becomes true and the loop ends.

continue and break

Example for continue

for i in [series]
do
    command 1
    command 2
    if (condition) # Condition to jump over command 3
            continue # skip to the next value in "series"
    fi
    command 3
done

Example for break

for i in [series]
do
    command 4
    if (condition) # Condition to break the loop
    then
            command 5 # Command if the loop needs to be broken
            break
    fi
    command 6 # Command to run if the "condition" is never true 
done

Looping over an array

for loop:

arr=(a b c d e f)
for i in "${arr[@]}";do
    echo "$i"
done

Or

for ((i=0;i<${#arr[@]};i++));do
    echo "${arr[$i]}" 
done

while loop:

i=0
while [ $i -lt ${#arr[@]} ];do
    echo "${arr[$i]}"
    i=$(expr $i + 1)
done

Or

i=0
while (( $i < ${#arr[@]} ));do
    echo "${arr[$i]}"
    ((i++))
done

Loop break

Break multiple loop:

arr=(a b c d e f)
for i in "${arr[@]}";do
    echo "$i"
    for j in "${arr[@]}";do
        echo "$j"
        break 2
    done
done

Output:

a
a

Break single loop:

arr=(a b c d e f)
for i in "${arr[@]}";do
    echo "$i"
    for j in "${arr[@]}";do
        echo "$j"
        break
    done
done

Output:

a
a
b
a
c
a
d
a
e
a
f
a

Switch statement with case

With the case statement you can match values against one variable.

The argument passed to case is expanded and try to match against each patterns.

If a match is found, the commands upto ;; are executed.

case "$BASH_VERSION" in
 [34]*)
    echo {1..4}
    ;;  
  *)
    seq -s" " 1 4
esac

Pattern are not regular expressions but shell pattern matching (aka globs).

For Loop without a list-of-words parameter

for arg; do
    echo arg=$arg
done

A for loop without a list of words parameter will iterate over the positional parameters instead. In other words, the above example is equivalent to this code:

for arg in "[email protected]"; do
    echo arg=$arg
done

In other words, if you catch yourself writing for i in "[email protected]"; do ...; done, just drop the in part, and write simply for i; do ...; done.

Conditional execution of command lists

How to use conditional execution of command lists

Any builtin command, expression, or function, as well as any external command or script can be executed conditionally using the &&(and) and ||(or) operators.

For example, this will only print the current directory if the cd command was successful.

cd my_directory && pwd

Likewise, this will exit if the cd command fails, preventing catastrophe:

cd my_directory || exit
rm -rf *

When combining multiple statements in this manner, it's important to remember that (unlike many C-style languages) these operators have no precedence and are left-associative.

Thus, this statement will work as expected...

cd my_directory && pwd || echo "No such directory"
  • If the cd succeeds, the && pwd executes and the current working directory name is printed. Unless pwd fails (a rarity) the || echo ... will not be executed.
  • If the cd fails, the && pwd will be skipped and the || echo ... will run.

But this will not (if you're thinking if...then...else)...

cd my_directory && ls || echo "No such directory"
  • If the cd fails, the && ls is skipped and the || echo ... is executed.
  • If the cd succeeds, the && ls is executed.
    • If the ls succeeds, the || echo ... is ignored. (so far so good)
    • BUT... if the ls fails, the || echo ... will also be executed.

      It is the ls, not the cd, that is the previous command.

Why use conditional execution of command lists

Conditional execution is a hair faster than if...then but its main advantage is allowing functions and scripts to exit early, or "short circuit".

Unlike many languages like C where memory is explicitly allocated for structs and variables and such (and thus must be deallocated), bash handles this under the covers. In most cases, we don't have to clean up anything before leaving the function. A return statement will deallocate everything local to the function and pickup execution at the return address on the stack.

Returning from functions or exiting scripts as soon as possible can thus significantly improve performance and reduce system load by avoiding the unnecessary execution of code. For example...

my_function () {

    ### ALWAYS CHECK THE RETURN CODE

    # one argument required. "" evaluates to false(1)
    [[ "$1" ]]             || return 1

    # work with the argument. exit on failure
    do_something_with "$1" || return 1
    do_something_else      || return 1

    # Success! no failures detected, or we wouldn't be here
    return 0
}